Musannaf Ibn Abi Shayba 13:1471

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mansur, from Ibrahim, who said: If a *mukātab* (an enslaved person under a contract of manumission) dies while he has a debt, his masters shall be treated like the creditors regarding what has become due of his installments. If he has no installment that has become due, the creditors shall be prioritized, and they shall take their debt. If anything remains, it shall be for his masters until his *mukātabah* (contract) is completed. If anything remains after his *mukātabah* is completed, it shall be for his heirs.