Leviticus 25:53

Wages. Hebrew, "as a yearly hired servant shall he be with him." What was customarily given to a hired servant for a certain number of years, might be a rule to judge how much was to be paid for redemption. (Haydock) --- Thus if a man had engaged to serve 20 years for 100 sicles, and at the expiration of 10 years wished to redeem himself, he might do it for half that sum. Some think, that those Hebrews who had sold themselves to a Gentile, sojourning among them, could not take the benefit of the sabbatic year, (Exodus xxi. 6,) because Moses is silent on this head. But this argument is not satisfactory. (Calmet)