פירוש על קידושין 51:5

Rav limits the mishnah’s rule to a place where deeds are not normally written. But if deeds are normally written, then a deed must be written to acquire land.

The Talmud, based on a practice of R. Idi b. Abin, allows the purchaser to make a statement that allows him to retract but not the seller. Of course, the seller has to agree to this arrangement, but if he dies, the purchaser has a legal right to make it.