Musannaf Ibn Abi Shayba 13:1610

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from Isma'il, from al-Sha'bi, regarding a man who bought a slave and then emancipated him, then found him to have madness. He said: If the ailment was before the transaction, the seller shall return to the buyer the difference between [the value of] health and the ailment, and he shall put what he took into the like of him.