Musannaf Ibn Abi Shayba 13:3118

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, he said: Mufaddal ibn Muhalhal narrated to us, from Mutarrif, from al-Sha’bi, regarding a man who permits his slave to engage in trade, then sells him. He said: He [the master] is liable.