Musannaf Ibn Abi Shayba 13:192

20151. Abd al-Wahhab [ibn] 'Ata narrated to us, from Sa'id, from Qatadah, from Umar ibn Abd al-Aziz, that he used to see no harm in writing [a contract] for servants [to be sold] hand-to-hand. And he disliked that [if it were] on credit. And that is the opinion of Qatadah.