Musannaf Ibn Abi Shayba 13:613

Abu Bakr narrated to us, he said: Yahya ibn Yaman narrated to us, from Isra'il, from 'Abbad ibn Mansur, from Hammad, from Ibrahim, from 'Uthman, who said: The *mukātab* (a slave who has entered into a contract of manumission) is a slave as long as a dirham remains owed by him.