Nafi' (may Allah have mercy on him) says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) had a mukatab slave whose contract was 30,000. Then he came and said: I have become unable to pay. He said: Then cancel your contract. He said: I have become unable to pay; you erase it yourself. Nafi' (may Allah have mercy on him) says: I signalled to him to erase it, hoping that he would be freed. So the slave erased it. He had one or two sons. Ibn Umar (may Allah be pleased with them both) said: Keep away from my slave girl. Then Ibn Umar (may Allah be pleased with them both) later freed his son.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21754