Abd al-Rahman bin Abi al-Zinad (may Allah have mercy on him) narrates from his father (may Allah have mercy on him), and he from the jurists of the people of Madinah (may Allah have mercy on them), that for every animal that was injured they would assess the difference in value between (the animal) sound and injured. Every injured animal they would assess in this way. Isa bin Mina (may Allah have mercy on him) says: As for an injured slave, they would reckon (his wound) against his value on the day he was injured, just as the wound of a free man is reckoned against the blood money.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الغصب / 11529