Sayyiduna Jabir bin Samurah (may Allah be pleased with him) narrates from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that when the mukatab has paid half (of the price), he is not to be enslaved (again).
Note: Al-Qasim bin Abd al-Rahman's hearing from Jabir bin Samurah is not established. If it is authentic, his intent is that when he has come close to freedom, he should be given respite to earn and pay the remainder, rather than be returned to slavery.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21650