Musannaf Ibn Abi Shaybah 35768 — A free person whom a man buys

Ibrahim (may Allah have mercy on him) said: If the enemy captures one of the Muslims and then a trader buys him, that person shall strive to pay the trader the price for which he bought him. If they capture slaves belonging to the Muslims and then a trader buys them, and then their master finds them, he has the greater right to take them by paying the price. And if they buy one of the ahl al-dhimmah, he shall strive to pay the trader the price and gain his freedom.