Adam Clarke on Exodus 21:27

If he smite out his - tooth - It was a noble law that obliged the unmerciful slaveholder to set the slave at liberty whose eye or tooth he had knocked out. If this did not teach them humanity, it taught them caution, as one rash blow might have deprived them of all right to the future services of the slave; and thus self-interest obliged them to be cautious and circumspect.