Musannaf Ibn Abi Shayba 35:277

Abu al-Ahwas narrated to us, from Abd al-Aziz ibn Rufay’, from ‘Ata’ and Ibn Abi Mulaykah: They both said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sells a slave, then his wealth belongs to the seller, unless the buyer stipulates [otherwise]." He says: "I buy him from you, and his wealth [is included]." And it was mentioned that Abu Hanifah said: "If the wealth of the slave is more than the price, that is not permissible."