Mughirah (may Allah have mercy on him) says: I asked Ibrahim (may Allah have mercy on him): If a man dies and leaves a mukatab, and the heirs free the mukatab, whose right is the wala'? He said: It is the right of the deceased.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21751