Musannaf Ibn Abi Shayba 13:614

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Dawud ibn Abi al-Furat, from Ibrahim al-Sa’igh, from ‘Ata’, ‘Abdullah ibn ‘Ubayd ibn ‘Umayr, and Nafi’, they said: The *mukātab* (a slave under a contract of manumission) is a slave as long as there remains a dirham [of his debt] upon him.