Musannaf Ibn Abi Shayba 13:3259

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Hisham, from al-Hasan, regarding a man who buys a slave on the condition that he emancipates him, but then he does not do so. He said: If he emancipates him, [it is valid], otherwise he returns him.