When Moshe returns to HaShem to begin his defence of B'nei Yisrael, he opens by stating (32:31) "The nation has sinned a great sin; they have made for themselves a god of gold." Rashi, quoting the gemara (Berachos 32a, Yoma 86b) writes that Moshe's intention with this opening argument was, in fact, to place the blame on HaShem, so to speak, for having showered so much gold upon B'nei Yisrael. Rashi adds a parable that illustrates Moshe's argument. It is assumed that Moshe is referring to the loot that B'nei Yisrael were commanded to collect before leaving Egypt.
My father, z"l, once asked me that Rashi writes (15:22) that the spoils collected at Yam Suf when they Egyptian soldiers were washed onto the shore along with their horses and chariots was far greater than that which was collected before B'nei Yisrael left Egypt. If so, Moshe's argument is no longer valid. HaShem may have commanded B'nei Yisrael to collect the gold from the Egyptians in Egypt. But they were never commanded to loot the soldiers after they were washed on the shore of Yam Suf. How, then, can Moshe claim that HaShem was solely responsible for their wealth?
I believe the answer lies in the Vilna Gaon's understanding of the two episodes which I will summarize briefly. The gemara (Berachos 9a) relates that HaShem asked in the form of a request that B'nei Yisrael collect the silver and gold from the Egyptians. This was done in order to appease Avraham Avinu, so that he does not claim that the promise that his descendants would be subjugated was kept but the promise that they leave with a large bounty was not.
The GR"A asks that if this was in fact the proper fulfilment of the promise to Avraham, why was it done to "appease" him so that he does not raise a complaint? Avraham Avinu's reaction should not be the issue. It should be a matter of whether or not it is time to fulfill the promise. He answers that in truth, the exodus from Egypt was only ultimately complete at Yam Suf where the Egyptians truly got their deserved punishment for subjugating B'nei Yisrael. It was only then that HaShem's promise to Avraham that his descendants shall leave with a great bounty needed to be fulfilled. However, in case Avraham mistakenly viewed the leaving of Egypt as the ultimate redemption, HaShem had B'nei Yisrael collect a great bounty there before leaving to avoid any possible objections from Avraham.
According to this approach, although B'nei Yisroel were never commanded to collect the adornments from the horses of the Egyptians at Yam Suf, this too was part the fulfilment of the promise that HaShem made to Avraham Avinu 400 years prior. Therefore, Moshe Rabbeinu's defence was still valid.
Have a good Shabbos.