פירוש על קידושין 45:3

According to R. Meir, others may buy the slave’s freedom—they may pay to set him free. So the question is what are the circumstances? The first possibility is that the slave has not consented to be set free. But this should not work for R. Meir holds that it is not to the slave’s advantage to go free. And we also know that a person cannot cause disadvantage to another without his consent. Therefore this cannot refer to a case where the slave has not consented to be set free.