From: Rabbi Akiva Eiger
To: Rabbi Simcha Bunim Ginz (Eiger)
Rabbi Akiva Eiger rules that both fulfilling a condition on Shabbos that retroactively validates a kinyan and stipulating a kinyan to take effect automatically on Shabbos constitute rabbinic violations (shevus). The proof is drawn primarily from the Yerushalmi's discussion of the Kohen Gadol's substitute wife, which permits shevus in the Mikdash, implying such arrangements are prohibited outside it.
Source: Shu"t Rabbi Akiva Eiger §159