As Bnai Yisrael were about to leave Mitzrayim, Hashem told Moshe to instruct the people to take gold and silver from their Egyptian neighbors (שמות י"א:ב'). Rashi (שם) explains that Hashem was fulfilling a promise he had made to Avraham Avinu. After He told Avraham that his descendants would suffer greatly as slaves, He promised that they would "go forth with a great abundance of wealth" as free men (בראשית ט"ו:י"ד). Chazal tell us that this "great abundance of wealth" refers to the Torah which they were to receive at Har Sinai. But if this is so, why were Bnai Yisrael commanded to ask the Egyptians for actual gold and silver before leaving Egypt? According to Chazal, they were never promised monetary wealth, but the spiritual wealth of the Torah.
The Maggid of Dubno answered this question with the following story. A young man was once hired out to a wealthy merchant for several years on the understanding that at the end of his service, he would receive a bag of silver coins. When the years ended, it occurred to the master that one bag of silver was not enough payment for all the services that this young man had done for him. He therefore put the silver aside and wrote the young man a check for an amount many times greater than the silver had been worth. However, when the young man saw the check, he looked very disappointed and stuffed it into his pocket. He had been promised silver coins and now he felt cheated having received merely a piece of paper.
The next day, the young man's father went to the merchant's house and explained that although by giving him the check, he was being most generous to the young man, for which he thanked him, his son is still very young and does not understand the value of a check. He expected a bag of shiny silver coins and he got a plain sheet of paper instead. The father therefore asked the merchant if he would let him have at least part of his wages in silver. The Maggid of Dubno explained that in the same manner, Avraham might have come to Hashem and said that He is indeed being generous in giving the Torah to his descendants, but Bnai Yisrael, at the time of Yetzias Mitzrayim, are too young and inexperienced to understand the value of the Torah, and will view it as a worthless piece of paper. If they go out from slavery empty-handed, they will say that Hashem fulfilled only the part of His promise that they would become slaves, but not the part about them going out with a great abundance of wealth.
For this reason, Bnai Yisrael were commanded to take gold and silver from the Egyptians. This way, they would be leaving with tangible wealth; something they could appreciate at the time more than the Torah. It was only as they grew in wisdom that they came to understand that the true wealth they received was not the coins and jewels gathered in Egypt, but the Torah, Hashem's greatest gift, which has given us spiritual and moral richness to this day.