Abu al-Zubayr narrates from Sayyiduna Jabir (may Allah be pleased with him) that he said: "They (the masters) keep what they took from him," meaning: when he does not complete (his contract) and is returned to slavery, whatever (the master) took is his.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21759