Ishaq, the freed slave of Abdullah bin Umar (may Allah be pleased with them both), says that his father made a contract of manumission with Abdullah bin Umar (may Allah be pleased with them both) for 30,000. He became unable to pay, so he returned him to slavery, after he had paid half or nearly half. He asked him to free his children, who had been born during his contract of manumission, so he freed him and his children too, and also returned 1,500 dirhams to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21757