John Calvin Commentary Exodus 21:26

"And if a man smite the eye of his servant, or the eye of his maid, and destroy it; he shall let him go free for his eye`s sake." — Exodus 21:26 (ASV)

And if a man smite the eye. Since, in the sight of God, there is neither slave nor free man, it is clear that he sins as greatly who strikes a slave as if he had struck a free man.

Still, a distinction is made regarding the civil law and human justice, especially if anyone has inflicted a wound on his own slave. For here, a tooth for a tooth, or an eye for an eye, is not required; instead, the superiority that he has improperly abused is taken from the master.

In compensation for the injury, liberty, which is almost half their life, is given to the male or female slave. Thus, considering that it was his slave, the master is treated more leniently, as the severity of the punishment is mitigated in this way; while, in compensation for his dislocation or fracture, the slave receives what is more advantageous to him: namely, that, being set free, he should not be exposed to another’s cruelty.