Musannaf Ibn Abi Shayba 13:519

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Mansur, from Ibrahim and al-Sha'bi: They both said: There is no harm in selling the *wala'* (patronage) of the *sa'ibah* (emancipated slave) and gifting it.