According to the plain meaning, "great possessions" refers to the spoils of Egypt, silver and gold vessels, and garments. Indeed, the Gra (Vilna Gaon, in his commentary "Shenot Eliyahu" on Berachot) elaborated to prove that the main promise of the great possessions was not said about the time of the Exodus from Egypt, but rather about the time they were at the Yam Suf (Red Sea). All that they requested possessions from the Egyptians was only to appease the mind of Avraham Avinu (our father Abraham).