פירוש על קידושין 53:2

Rav explains that if the person only says “and write the deed” he can retract from his instruction to write the deed. But he cannot retract from the granting of the field itself. The transfer of the land has already occurred and cannot be taken back. However, if he stipulates “on condition” then the land is not transferred until the deed is written. Therefore, he can retract from both elements.