Musannaf Ibn Abi Shayba 13:1579

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Muhammad ibn Ishaq, from Nafi’, from Ibn ‘Umar, that a *mukātab* (a slave with a contract of manumission) of his became unable to pay, so he returned him to being a slave and kept what he had taken from him.