Musannaf Ibn Abi Shaybah

Hazrat Musa bin Ali (may Allah be pleased with him) bin Rabah narrates that I asked Hazrat Zuhri: A mukatab (contracted slave) married a free woman and had children from her. Then he bought a slave woman and had children from her as well, and then he himself passed away, while some amount of his contract of manumission remained unpaid. Who will strive to pay the remaining amount of the contract? He replied: The boy who was born from the slave woman will do so.