Musannaf Ibn Abi Shayba 13:1776

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from Mansur, from Ibrahim, regarding a slave who is between two men, and one of them emancipates his share: He said: He [the emancipator] must provide compensation if he has wealth. If he does not have wealth, the slave is made to work to earn his freedom [istas'a].