Musannaf Ibn Abi Shayba 13:1904

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from Shu'bah, from al-Hakam, who said: If a young slave is in the tax-paying status (dharibah) and he makes a purchase, then it is upon his neck (i.e., he is personally liable for it).