Kinyan Di-rabanan For A Di-oraisa

The Mishna Brura [סי' תרנ"ח] cites the Machane Ephraim who says that one should not buy his esrog on credit because the kinyan of meshicha is only di-rabbanan and that doesn't work for a di-oraisa [HUUUUGE topic in the poskim. We have written about it elsewhere BS"D] so one must give money.

However, the mishna in Shabbos [148] says that when Erev Pesach falls on Shabbos you give your garment to the seller [as a security], take the Pesach and make the cheshbon after Shabbos and Yom Tov. Now, the Pesach must belong to you and thus requires a proper kinyan [especially according to the Griz who says that the din of minui necessitates a baalus of mamon], yet here there is no monetary kinyan [it is Shabbos]. How then does this di-rabanan-dike kinyan [of meshicha] work for the di-oraisa??

[Rav Yechiel Dzimitrovsky Shlita]