Musannaf Ibn Abi Shayba 13:2620

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, regarding a man who said to another man: "If I sell you my slave, then he is free." And the other said: "If I buy him, then he is free." He said: He is emancipated from the wealth of the seller, because he broke the oath before him.