Ibn Ezra on Exodus 21:26/אבן עזרא על שמות כ״א:כ״ו

1AND IF A MAN SMITE. God commanded that a bondman should go free because of his eye or tooth to prevent masters from cruelly striking their servants with terrible blows. Thus if the master destroys his bondman’s eye or even his tooth, then the bondman will pass from his control free of charge and the master will forfeit his property.