Musannaf Ibn Abi Shayba 13:607

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Isma’il, from al-Sha’bi, and from Sufyan, from Ibn Abi Najih, from Mujahid, from Zayd, he said: The *mukātab* (a slave under a contract of manumission) is a slave as long as there remains a dirham [owed] by him.