Abu al-Zubayr heard Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) saying concerning the mukatab (a slave who has contracted to buy his freedom) who has paid the first part of his contract and then becomes unable to continue: is he to be returned to slavery? He said: "His master has the greater right to the condition that he stipulated."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21758