Musannaf Ibn Abi Shaybah

Hazrat Zuhri (may Allah have mercy on him) says that if a man makes his slave a mukatab (contracted for freedom) for a certain period in exchange for some dirhams, and then before the appointed time says to him, "Pay quickly and I will reduce the amount of the contract," there is no harm in this. He says that I have not seen anyone except Hazrat Ibn Umar (may Allah be pleased with them both) who disliked this; indeed, he used to dislike it, but he considered it permissible in exchange for goods.