Nafi' (may Allah have mercy on him) narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he made a contract of manumission with his slave, whose child and umm walad (slave mother of his child) were also included. He came to Sayyiduna Ibn Umar (may Allah be pleased with them both) and said: I have become unable to pay; take back your contract. Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I will not accept it from you until you bring them. When he brought them, he returned them to slavery. Then, one or three days later, he freed them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21755