Sforno on Exodus 21:20/ספורנו על שמות כ״א:כ׳

1נקום ינקם, the blood of the slave. The master is certainly not permitted to administer such a cruel blow even though he owns this human being. He is allowed to administer physical punishments as we know from Proverbs 29,19 בדברים לא יוסר עבד, “a slave does not respond to mere oral chastisement.”