פירוש על ויקרא 24:18

AND HE THAT SMITETH A BEAST MORTALLY SHALL PAY FOR IT; ‘NEFESH’ (LIFE) FOR ‘NEFESH’ (LIFE). This does not mean a real animal, that he must buy him an animal similar to his one [that was killed], but rather it means that he is to give him monetary compensation equal to the damage he caused him. Similarly, breach for breach, eye for eye⁵³⁸Verse 20. [also means monetary compensation], according to the opinion of our Rabbis.⁵³⁹Baba Kama 83 b.

מכה נפש בהמה, so that the animal dies.

Whoever kills an animal shall pay for it. It should have said ישלם [instead of ישלמנה]. This is the source for the halachic derivation (Baba Kama 10b): He shall complete it [ישלימנה — he pays for the difference after the owner deducts the value of the carcass]. According to the plain meaning: He should pay minimally, according to the animal’s value; for besides the differences in the animal’s skill or work, there is also an evaluation based on its lineage — someone who had the horse upon whom the king rode … he is obligated to pay only its minimum worth.