Gemara:


Ein Ayah: The main higher intention of the exodus with great riches was to elevate the spirit of the nation, which had been in the lowly status of slavery for many years. In that state their spirits were lowly, and they lacked the initiative to desire greatness. Therefore, it was fitting that they train themselves to desire big things so that they would come to aspire for greatness in the levels of the spirit and the higher personal characteristics.
In order that one should not think that the intention was to aspire for the love of gold and silver, the matter was not presented as a command but as a request, so that the matter would come out in the optimal manner. Their lowly spirit would be elevated when they saw themselves surrounded with riches and with it they would know that this was not the ultimate purpose because all matters of intrinsic value received a command or a warning. This one was presented as a favor, lest Avraham have a claim [that the promise of slavery was fulfilled and that of riches was not].
The matter of Avraham’s claim must also be seen in this light. His whole goal had been to establish a nation that recognizes Hashem and informs the world, by its existence and its behavior, of His great name. That is the way Avraham acted in his lifetime, as a unique individual. In order to have an impact on many nations, one needs the greatness of spirit and aspirations even for such things as ownership and possession of material wealth. The commercial world is a setting where many peoples are bonded one with the other and one learns from the ways of the other. Therefore, by means of the love of money, which causes one to buy and sell and, when things go well, brings on "prosperity through justice" it also causes the desired goal of Israel spreading the light of Hashem in the world. It would be different if Bnei Yisrael were in a less ambitious mindset and sufficed with being only shepherds and farmers. Then they would have nothing to do with other peoples and would be unknown among the nations. How then would the light of Hashem spread in the world?
Avraham knew that his descendants would have to undergo the purification cauldron to burn out their impurities and get them used to the subservience that is necessary to function under the yoke of Torah and mitzvot. Therefore, Avraham was interested that his goal of global recognition of Hashem be met. This could be done by means that his descendants would, after extrication from the lowliness of slavery, get used to an elevated spirit and the aspiration for lives of international social interaction that accompany the desire for increased property.
This was explained nicely with a parable of a prisoner who awaited getting out of jail and did not want to wait longer in order to receive a lot of money. Israel, from the perspective of their lowly status, were not able to imagine a greater happiness than to be extricated from slavery and become independent. Therefore, they had to be requested to seek enrichment in order for the final goal to be reached. It is true that they were anyway going to receive the spoils found on the banks of the Red Sea, which, the Rabbis tell us, exceeded that which they took from the Egyptians before. However, maybe the Egyptians pursued them until the Red Sea because Bnei Yisrael took their money. Alternatively, it is possible that the riches prior to the splitting of the sea were needed to elevate the spirit so that they could reach the level of perceiving the Divine at Yam Suf, as the Rabbis say, that a maidservant on the sea saw more than great prophets.