Musannaf Ibn Abi Shaybah

Yahya bin Adam says that I asked Hasan bin Salih and Sharik about a person who mortgages a slave and then sets him free? They replied that setting him free is permissible, and Sharik says that the slave will strive to pay off the debt of the mortgagee, and Hasan says that striving for the mortgagee is not the responsibility of the slave.