Musannaf Ibn Abi Shayba 13:2262

Abu Bakr narrated to us, he said: Abd al-Salam ibn Harb narrated to us, from Ata ibn al-Sa'ib, from Tawus, that he saw no harm in a *mukātab* (a slave under a contract of manumission) saying to his master: "Reduce [the amount] for me, and I will hasten [payment] for you."