Musannaf Ibn Abi Shayba 13:620

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Hisham, from al-Hasan, that he said regarding a *mukātab* (an enslaved person under a contract of manumission) who became unable to pay and had already paid a portion of his manumission contract, and they had stipulated conditions upon him, so he is returned [to slavery]: He said: If he has paid half, then he is a debtor.