Musannaf Ibn Abi Shayba 13:617

Abu Bakr narrated to us, he said: Abd al-Wahhab al-Thaqafi narrated to us, from Yahya ibn Sa'id, from Urwah, that Marwan used to judge that if the *mukātab* (a slave under a contract of manumission) had paid half of his manumission contract, then it is a debt that is to be pursued. I mentioned that to Abd al-Malik ibn Marwan, but he refused to accept it.