Musannaf Ibn Abi Shayba 13:1459

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Hajjaj, from ‘Amr ibn Shu’ayb, from his father, from his grandfather, from the Prophet (peace be upon him), he said: "If he enters into a contract of manumission (kitabah) with his slave for one hundred awqiyah, and he pays it off until ten awqiyah remain, then he becomes unable (to pay), he is returned to slavery."