פירוש על קידושין 51:8

R. Hamnuna cites a baraita which explains the mishnah. This baraita equates between selling and given—in both cases the transaction is valid even before money was given.

R. Hamnuna distinguishes between one who sells something because it is poor quality and one who sells something because he needs the cash. If he is selling it because it is poor quality, then he really wants to get rid of it, and the sale is valid as soon as the deed is signed. However, if he is selling for other reasons, he may retract until he receives the cash.