Musannaf Ibn Abi Shayba 13:2649

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from Ibrahim—or from Hammad, from Ibrahim—he said: There is no harm in the *mukātab* (a slave who has entered into a contract of manumission) being sold, if there remains [a portion] of his *mukātaba* (contractual payment) due, to one who will purchase him and guarantee his manumission. But he is not to be sold into slavery.