The commercial expedition of Muhammed to Syria turned out to be the prelude of his marriage with Khadija.

The translator and commentator of Quran Majid, Yusuf Ali, poses the following rhetorical question in this context:

"Can we wonder at Jacob's re-union with Joseph, or that of Moses with Aaron, or of Muhammad Mustafa with the Lady Khadija?"

No. We cannot. It was the decree of Allah that two of his slaves - Muhammed and Khadija - should be united in marriage, and they were.

It is reported that one of the close friends of Khadija was a high-born lady of Makka called Nafisa (or Nufaysa) the daughter of Munyah. She was aware that Khadija had turned down many proposals of marriage. At first she wondered if there was any man in Arabia who would come up to the standards set by her. She had discussed the matter many times with Khadija. Finally, she had one more discussion with her which convinced her that she (Khadija) was not impressed by any man's wealth or rank or power. What really impressed Khadija, her friend gathered, was character - a sterling character. Khadija admired only a man of ethical and moral principles.
Nafisa (or Nufaysa) also happened to know that there was such a man in Makka and his name was Muhammed.
It is reported that one day Muhammed was returning home from the Kaaba when Nafisa stopped him, and the following exchange took place between them:
Nafisa: O Muhammed, you are a young man and you are single. Men who are much younger than you, are already married; some even have children. Why don't you marry?
Muhammed: I cannot afford to marry; I am not rich enough to marry.
Nafisa: What would be your response if you could marry a woman of beauty, wealth, status and honor, notwithstanding your present poverty?

Muhammed: Who could be such a woman?
Nafisa: Such a woman is Khadija the daughter of Khuwayled.
Muhammed: Khadija? How is it possible that Khadija would marry me? You know that many rich and powerful princes and chiefs of tribes proposed to her, and she rebuffed them all.
Nafisa: If you are agreeable to marry her, you just say so, and leave the rest to me. I shall arrange everything.

Muhammed wished to inform his uncle and guardian, Abu Talib, about Nafisa's demarche, and to consult him in the matter before giving her an answer.
Abu Talib knew Khadija as well as he knew his own nephew. He welcomed Nafisa's suggestion. There was no doubt in his mind that Muhammed and Khadija would make the ideal couple. He, therefore, gave his blessings to the proposal of their marriage. Thereupon, Muhammed told Nafisa that her suggestion
was acceptable to him and that she had the authority to negotiate, on his behalf, his marriage with Khadija.
Once Abu Talib had approved the match, he sent his sister, Safiya, to see Khadija, and to talk with her about the proposed marriage. In the meantime, Nafisa had already done the "groundwork," and Khadija was expecting a visitor from the house of her future in-laws. She cordially received Safiya, entertained her, and told her that she (Khadija) had selected her (Safiya's) nephew to be her (Khadija's) life-partner without any preconditions and reservations. Safiya was very happy with the success of her embassy. Before she left the house, Khadija gave her an elegant robe which she accepted with many expressions of joy and gratitude.
Abu Talib then decided to comply with the traditional formalities of marriage. He bought gifts for Khadija, and took his brothers, Abbas and Hamza, with him to her house to formally present to her the proposal of the marriage of his nephew with her. Khadija accepted the gifts that Abu Talib had brought, and of course she accepted the proposal of marriage. The two parties immediately fixed a date for the auspicious wedding.
Abu Talib himself took charge of the preparations for the marriage of his beloved nephew. For the blessed occasion, he brought out all the heirlooms of the family and the sacred relics of his forefathers. These included the cloak and the staff of Abdul Muttalib, the late chief of Bani Hashim. The bridegroom put on the cloak and held the staff in his hand. Abu Talib put the black turban of his clan on his (the bridegroom's) head, and a ring of green agate on his finger. The ring, at one time, had belonged to Hashim bin Abd Manaf bin Qusayy.

The wedding party was made up of all the chiefs of Quraysh and the lords of Makka. The bridegroom rode a proud and prancing horse, and the young warriors of Bani Hashim brandished gleaming swords high above their heads as they escorted him from the house of Abu Talib to the house of Khadija. The women of the clan had gone ahead of the bridegroom, and were already being entertained in the house of the bride.
Khadija's house was illuminated by myriads of lamps. Inside the house, chandeliers hung on golden chains from the ceiling, and each chandelier held seven lamps. The guests arrived in the amber dusk. The chief steward of Khadija's estates had formed a committee for the reception of the bridegroom and the distinguished guests. The members of this committee conducted them inside the house through a high-arched entrance to a rectangular hall whose walls were panelled with tiles and whose ceiling was gilded. They made themselves comfortable on rugs and cushions.
For this special occasion, Khadija had ordered a special outfit to be made for all her domestics - male and female. Men were handsomely arrayed in spangled turbans, scarlet tunics, and black sashes around their waists. Attached to their turbans were silk tassels of ivory hue. The girls were wearing decor-blending costumes which dripped with gold and spangles. They were wearing coronets on the head and ropes of pearls and rivers of crystals. Their hair, cascading from the head to the shoulders and from the shoulders down to the waist, was braided with pearls.
The decor of the chamber of the bride was exquisite and was in fact, unsurpassable in taste and skill. The hangings of silk and brocade in many delicate tints, draped the walls; and a white velvet carpet covered the floor. The smoke of incense rose from a goblet of silver sparkling with diamonds, blue sapphires and balas rubies.
Khadija, the bride, sat on a high dais placed under a richly embroidered canopy. She looked radiant and resplendent like the rising sun itself. On her head she was wearing a crown of gold and pearls of amazing orient and beauty. Her dress, in subtle
shades of crimson and green, was shot with gold, and was set with pearls and emeralds. There were two maids in personal attendance on her; each was wearing a diadem of gold, an amethyst silken dress, and jewel-studded slippers.

When all the guests had taken their seats, Abu Talib, the guardian of the bridegroom, rose to read the sermon of marriage as follows:

All glory and all praise to Allah, the Creator of Heavens and earth, and all thanks to Him for all His blessings, bounties and mercy. He sent us into this world in the posterity of Ibrahim and Ismael. He put us in charge of the Mosque and made us guardians of His House, the Kaaba, which is a sanctuary for all His creatures.
After this exordium, Abu Talib continued:
My nephew, Muhammed ibn Abdullah ibn Abdul Muttalib, is the best individual in all mankind in his intelligence, in wisdom, in purity of lineage, in purity of his personal life, and in distinction of family. He has all the markings of a man destined to be great. He is marrying Khadija the daughter of Khuwayled against a meher of four hundred pieces of gold. I declare Muhammed and Khadija husband and wife. May Allah bless them both, and may He be their Protector.
In his sermon, Abu Talib declared that the Bani Hashim were the heirs of Ibrahim and Ismael, and were the carriers of their heritage. They were, therefore, uncontaminated by idolatry.

When Abu Talib concluded his sermon, Waraqa bin Naufal rose to read the marriage sermon on behalf of the bride. He said:
All praise and glory to Allah. We testify and we affirm that the Bani Hashim are just as you have claimed. No one can deny their excellence. Because of their excellence, we cherish the marriage of Khadija and Muhammed. Their marriage unites our two houses, and their union is a source of great happiness to us. O Lords of Quraysh, I want you to be witnesses that I give Khadija in marriage to Muhammed ibn Abdullah against a meher of four hundred pieces of gold. May Allah make their marriage a happy one.
(M. Shibli, the Indian historian, says in his Seera that the meher of Hadret Khadija was five hundred pieces of gold).
Amr bin Asad, the aged uncle of Khadija, also spoke on the occasion, and he affirmed, in his own words, what Waraqa b. Naufal, had said. And it was he who, as guardian of the bride, gave her away to Muhammed ibn Abdullah.

Abu Talib paid the meher for his nephew.

Edward Gibbon

At home and abroad, in peace and war, Abu Talib, the most respected of Mohammed's uncles, was the guide and guardian of his youth; in his 25th year he entered into the service of Khadija, rich and noble widow (sic) of Mecca, who soon rewarded his fidelity with the gift of her hand and fortune. The marriage contract, in the simple style of antiquity, recites the mutual love of Mohammed and Khadija; describes him as the most accomplished of the tribe of Koreish; and stipulates a dowry of twelve ounces of gold and twenty camels, which was supplied by the liberality of his uncle.
(The Decline and Fall of the Roman Empire)

Washington Irving

Khadija was filled with a lively faith in the superhuman merits of her youthful steward, Mohammed. At her nuptials, Haleema, who had nursed Mohammed in his infancy, was summoned and was presented with a flock of forty sheep.
(The Life of Mohammed)
All the guests congratulated Muhammed Mustafa on his wedding and expressed their best wishes for his happiness. They also congratulated his uncle, Abu Talib, on the auspicious occasion. Both thanked their guests cordially.
When these ceremonies were over, the major-domo ordered the slaves to spread out the banquet. The banquet was a gustatorial extravaganza such as no one had ever seen in Makka. The guests feasted upon delicacies each of which was a masterpiece of the culinary art. They slaked their thirst with delectable drinks laced with lotus nectar.
After the feast, each guest was invested with a robe of honor, in conformity with the ancient custom of the Arabian aristocracy.

Presently, the major-domo announced that the bride was ready to depart. A richly-caparisoned she-camel, carrying a white pavilion on her back, was waiting at the gate of the house. All the guests gathered in the foyer to see the bride being escorted to the gate. Her maids assisted her in climbing into the bridal pavilion.

...EMBARK YE ON THE ARK, IN THE NAME OF ALLAH, WHETHER IT MOVE OR BE AT REST! FOR MY LORD IS, BE SURE, OFT-FORGIVING, MOST MERCIFUL."
(Quran Majid. Chapter 11; verse 41)

One of the maidservants sat in the pavilion with the bride. Resting upon her head was a floral tiara, and her hair was threaded with blue ribbons and strands of lustrous pearls. She was wearing bracelets of agate, coral and rock crystal, and she held a jewelled fan in her hand.
A team of Nubian slaves carrying flambeaus, marched in front and on the right and the left sides of the she-camel.
The bridegroom also mounted his horse, and he, his uncles, the young men of Bani Hashim and their guests, returned to the house of Abu Talib in the same panoply as they had gone earlier that day to the house of the bride.
When this torch-lit procession arrived at the house of Abu Talib, his wife and sisters assisted the bride in dismounting from the she-camel. A chamberlain held a parasol of white silk over her head, and conducted her into the inner apartments of the house.
And say:

"O MY LORD! ENABLE ME TO DISEMBARK WITH THY BLESSING: FOR THOU ART THE BEST TO ENABLE (US) TO DISEMBARK"
(Quran Majid. Chapter 23, verse 29)

Everything went off with perfect precision. Coordination was superb from beginning to end.

The marriage of Muhammed and Khadija had brought happiness to everyone but the happiness of Abu Talib knew no bounds. He had been very anxious that his nephew should have a good wife. This anxiety turned into pure and undiluted joy when his nephew and Khadija were married. There could not have been a better match. Abu Talib thanked Allah for the new happiness he had found, and his happiness was shared by his brothers, Abbas an~ Hamza, and all other members of the clan of Hashim.

Three days after the marriage, Abu Talib made arrangements for a banquet to mark the occasion called since then "the feast of walima." He dazzled the whole city by his liberality. At the feast, every resident of Makka was his guest. Muhammed, the bridegroom, was himself welcoming the guests into the house. He himself, his uncles, his cousins and all the young men of Bani Hashim, were the proud hosts. The banquet lasted for three days. Years later, Islam made the feast of walima a "memorial" to the banquet of Abu Talib at the marriage of Muhammed and Khadija, instituting it as a tradition of all Muslim marriages. Abu Talib was the first man to arrange it. Before the marriage of Muhammed and Khadija, the feast of walima was not known to anyone in Arabia.
Abu Talib must have wished that his beloved brother, Abdullah and his wife, Amina, may Allah bless them, were also present to witness and to bless the marriage of their son, and to share his (Abu Talib's) happiness. But even if Abdullah and Amina had been present, the marriage of their son could not have been celebrated with more pomp and pageantry than it was with Abu Talib as his (Muhammed's) guardian.

Next it was Khadija's turn to show generosity and hospitality. Generosity and hospitality were her old "addictions." And what occasion could be more appropriate or propitious for her than her own marriage to satisfy this propensity? She, therefore, ordered her major-domo to make arrangements for the most elaborate banquet in the history of Makka.
It was a banquet that was truly memorable. Even the beggars of Makka and the wandering tribesmen and women were not
excluded from the list of guests. They feasted on delicacies which they had never seen before. Those Arabs of the desert who had never tasted anything but brackish or rank water all their lives, drank rose water as the guests of Khadija. For many days the guests - rich and poor, high and low, lord and lackey, young and old - were fed in her house. To the poor guests, Khadija gave pieces of gold and silver and clothes, and she filled the houses of many widows and orphans with the necessities of life which they didn't have before.

Khadija had spent many years of her life waiting for the ideal man to come. Her long wait was at last rewarded when Muhammed came along, and they were united in holy wedlock.
The marriage of Muhammed and Khadija was the first and the last of its kind in the world. It was the only marriage in the whole world which abounded in heavenly blessings as well as material blessings. It was a marriage which was immeasurably and incalculably rich in the blessings of both the heaven and the earth.
It is entirely probable that in Arabia, no woman ever brought so much dowry with her into the house of her husband as Khadija did. It included slaves, slave girls, real estate, pasture lands, herds of camels and horses, flocks of goats and sheep, and her personal outfit of rich and rare fabrics, accessories, priceless heirlooms, ornaments, precious metals, precious stones and masses of gold and silver coins.
This dowry, unprecedented as it was for its quality and its quantity, was not a gift to Khadija - the bride - from her uncles or from her brothers. It was the product of her own efforts. She had produced it by her own diligence, industry, prudence, and foresight.
But these were not the only riches that Khadija brought with her. She also brought the riches of heart and mind, and these were immeasurable and inexhaustible. In the years to come she immeasurably enriched the life of her husband with these gifts.

Once Khadija was married, she appears to have lost interest in her mercantile ventures and in her commercial empire. Marriage changed the character of her dedication and commitment. She had found Muhammed Mustafa, the greatest of all treasures in the world. Once she found him, gold, silver and diamonds lost their value for her. Muhammed Mustafa, the future Messenger of Allah and the future Prophet of Islam, became the one object of all her affection, attention and devotion. Of course, she never lost her genius for organization, but now instead of applying it to her business, she applied it to the service of her husband. She reorganized her whole life around the personality of Muhammed Mustafa.
(Khadija could not wind up her wide-ranging commercial operations abruptly. She had to phase them out. By degrees, therefore, she phased out the export-and-import business which her father had founded.)
In the years following his marriage, Muhammed travelled again with Khadija's caravans to Syria. M. Shibli, the Indian historian, says that he also went to Yemen. Wherever he went, he made profits. Khadija also recruited other managers who sold her merchandize or bought merchandise for her, and they too made profits for her. But the emphasis had shifted; instead of expanding her business as she had done in the years before her marriage, Khadija began, gradually, to curtail her commitments until all her merchandize was sold, and she had recovered all her investments.