Musannaf Ibn Abi Shayba 13:1458

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, from Hajjaj, from al-Hakam, from Ibrahim, from Shurayh, that he used to return the *mukātab* (a slave who has a contract of manumission) if he became unable [to pay], and he would not grant him a delay.