Вайикра 24:18 Комментарий: Ibn Ezra, Ramban, Rashbam & Rav Hirsch

However, if he strikes … a beast then he must only make restitution . The phrase a life for a life is thus used in two different senses to refer to each of the previous two verses.

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.