Musannaf Ibn Abi Shayba 13:2137

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, he said: I asked al-Hasan ibn Salih and Sharik about a man who mortgages his slave, then emancipates him. They both said: His emancipation is valid. Sharik said: The slave must work (to pay off the debt) for the mortgagee. Al-Hasan ibn Salih said: There is no obligation of work upon him.