Musannaf Ibn Abi Shayba 13:1475

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hasan ibn Salih, Sufyan, and Ibn Abi Layla: They used to say: If he dies and he owes a debt, what is upon him becomes due. So the master is included with the creditors for the entire amount of the *mukātaba* (contract of manumission).