Ibn Jurayj (may Allah have mercy on him) says: I said to Ata' (may Allah have mercy on him): "If a mukatab dies while his children are free, and he leaves more wealth than his kitabah?" He said: "It will be ruled that the amount remaining be paid, and the rest be given to his sons." I said: "Has any such report reached you?" He said that Sayyiduna Ali (may Allah be pleased with him) used to judge in this way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21680