Musannaf Ibn Abi Shayba 13:615

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from al-Shaybani, from al-Sha’bi, who said: Abdullah said: "When the *mukātab* (a slave under a contract of manumission) has paid off [the price of] his neck, there is no return for him into slavery."