Musannaf Ibn Abi Shayba 26:589

Abu Dawud al-Tayalisi narrated to us, from Shu'bah, who said: Mansur informed me, from Ibrahim and al-Sha'bi, that they both said: There is no harm in selling the *wala'* (patronage rights) of the *sa'ibah* (a freed slave whose master has renounced all claims to him), or gifting it.