Musannaf Ibn Abi Shayba 13:3095

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, he said: If he says, "If I do such and such, then my slave is free," then he sells him, and then he does it, he said: There is nothing upon him.