פירוש על קידושין 118:6

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.

Rava solves the difficulty on R. Yohanan by positing that the case in the baraita is one in which the owner didn’t really annul the agency, but gave terumah on his own, before the agent did. By doing so, he is effectively nullifying the agent’s agency.