It is narrated from Sayyiduna Ali (may Allah be pleased with him) that if a slave (who has a contract of manumission) becomes unable to pay, he is to be made to work (to earn the amount) for two years. If he pays off his contract, he is free; otherwise he is to be made a slave again.
The first chain from Ali (may Allah be pleased with him) is weak, and the narration of Khilas from Ali (may Allah be pleased with him) is not authentic according to the scholars of hadith. If it is authentic, it is understood as a kindness on the part of the master; and if he does not wait, the slave is returned to slavery. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21761