Thurs Gitten 11

The Chachumim says, that once you gave the freeing-slave document to your messenger to give to the slave, he goes free right away. We say that the messenger can acquire it for the slave. From here the Gemara wants to prove that someone can grab something for a friend (that he's entitled to), even if it's detrimental to others that are also entitled to it. (For example, if a borrower died, and he only has enough money for one of his creditors, so a third person can acquire it for one of the creditors, although it will be detrimental to the second creditor, that cannot grab it for himself anymore.)

Tosfos explains why the slave is entitled to his freedom, although the master is never in obligation to free him. He also explains why, if we consider the slave entitled to his freedom, that it's considered detrimental to the master, since the slave is entitled to his freedom.

The explanation is: although the master is never obligated to free him, the very fact that he considers freeing him shows that the slave did him great favors to the point that the master feels morally obligated to free him. Even so, since he's not legally obligated to free him, we consider it as it's detrimental to the master.