Ibn Jurayj (may Allah have mercy on him) says that Ata' (may Allah have mercy on him) said: When a mukatab is injured, he has the right of retaliation (qisas); and Amr bin Dinar said the same. Ibn Jurayj (may Allah have mercy on him) said: Is that because it is his property, which he is responsible for safeguarding just as he safeguards his property? He said: Yes.
Imam al-Shafi'i (may Allah have mercy on him) said: It is as Ata' and Amr bin Dinar (may Allah have mercy on them both) said: The compensation for an offence against him is part of his own property; his master may not take it in any circumstance, unless he dies before paying off his contract.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21745