Musannaf Ibn Abi Shayba 13:1901

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Hammad, regarding a slave who was between two men, and one of them emancipated him. He said: The *wala’* (right of patronage) is between them, meaning if the slave is made to work to pay for his freedom.