Question

Hello Rabbi,
 
According to Rashi on Bava Metzia 10a, s.v. “he did not acquire,” the conclusion that one cannot acquire on behalf of a third party in a case where it harms others applies only when the acquirer was not appointed as an agent. But Tosafot, s.v. “one who seizes for a creditor,” and many medieval authorities, disagree and hold that the conclusion applies even when there was an agency appointment. The difficulty is from the passage in Ketubot 84a, where an agent was appointed, and nevertheless it says that it is forbidden to seize in a case where it harms others.
The Bach, in Choshen Mishpat siman 105, section 1, tries to reconcile Rashi and says that the passage in Ketubot is speaking where the agent was not appointed in writing, and then the sender can retract. Whereas Rashi means only a case where there was a written appointment.
And I ask: if the sender canceled the agency and the agent did not know, then it turns out retroactively that the seizure was not valid. That is true of any agency arrangement, and nevertheless such agency is permitted. What is different about one who seizes on behalf of a creditor?