Musannaf Ibn Abi Shayba 13:1456

Abu Bakr narrated to us, he said: Waki’ and Ibn Abi Za’idah narrated to us, from Aban bin Abdullah al-Bajali, from ‘Ata’, that Ibn ‘Umar made a *kitabah* (contract of manumission) with a slave of his for one thousand dinars. He paid it, except for one hundred, so he returned him to slavery.