Musannaf Ibn Abi Shayba 13:824

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Mughira, from Ibrahim, and from Ibn Abi Khalid, from al-Sha’bi, they both said: If he buys her and stipulates her manumission, they both did not consider her to be sound (i.e., the condition is not valid/binding).