Musannaf Ibn Abi Shayba 13:579

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-A’mash, from al-Sha’bi, regarding a man who entered into a *kitabah* (contract of manumission) with two of his slaves, and one of them died. He said: The portion [of the debt] corresponding to him is lifted from him.