Commentary for Kiddushin 45:6

The Talmud now introduces a solution that it immediately rejects. We might have thought that a slave can acquire himself through a deed even through his own agency, and not exclusively through his own agency. The innovation here would be that his freedom and his ability to acquire arrive at the same moment, such that at the same moment he is set free, he is also able to acquire the document. The problem is that there is a baraita in which R. Meir explicitly states that a slave acquires himself by document only through his own agency, not if the document is accepted by others.
Thus we now no longer have an acceptable situation for the Mishnah. If the slave did not consent, then why can others set him free by paying for his freedom? And if he did consent, then why can’t others accept a document of manumission on his behalf?