Musannaf Ibn Abi Shaybah

Hazrat Sha’bi (may Allah have mercy on him) was asked: If a person buys a slave and sets him free, and then it turns out that the slave is insane? He (may Allah have mercy on him) replied: If this illness existed before the transaction, then the seller will return to the buyer the difference between the price of an insane slave and a sound slave, and what he has taken, he will keep in its equivalent.