Musannaf Ibn Abi Shaybah 22924 — A man who buys a slave woman

Al-Sha'bi (may Allah have mercy on him) was asked: If a person buys a slave and frees him, and then the slave turns out to be insane? He (may Allah have mercy on him) said: If this illness existed before the transaction, the seller shall return to the buyer the difference between the value of an insane slave and a sound slave, and he shall put what he has taken toward the like of it.