Commentary for Kiddushin 117:13

The case here is one who sends an agent to separate terumah but then annuls the agency. If he does so before the agent separates the terumah, the separation is not valid. But here is a case of speech annulling speech. This seems to support R. Yohanan.

Here Resh Lakish retreats from his position a bit. Speech can annul speech. But the case of kiddushin was more than just speech. The woman received money. Since she accepted it for the sake of kiddushin, she cannot retract because this is like an action.