Musannaf Ibn Abi Shayba 13:3415

Abu Bakr narrated to us, he said: Yahya ibn Ya'la al-Aslami narrated to us, from Isa ibn Musa, who said: A man asked al-Hakam about a *mukātab* (a slave with a contract of manumission) between two men, one of whom emancipated him. He said: "It is merely property that he gifted to him; there is nothing upon him."