Deuteronomy 15:17

then thou shalt take an awl and thrust it through his ear unto the door, and he shall be thy servant forever. And also unto thy maid-servant thou shalt do likewise. Cf Exodus 21:5. This covered all the cases in which freedom might have been a hardship to the servant concerned, because the latter might have been unable to find employment elsewhere and would therefore once more have been reduced to extreme poverty.