Musannaf Ibn Abi Shayba 13:2265

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, that he said regarding a man who has a *kitabah* (contract of manumission) with his slave for a dirham to a specified term, and he says to him before the term arrives: "Pay me early and I will waive [a portion] for you": He saw no harm in it. He said: And I have not seen anyone who disliked it except Ibn Umar, for he used to dislike that unless it was for goods.