Leviticus 25:50-54

50The one who purchased him is to calculate the time from the year he sold himself to him until the Year of Jubilee. The price of his sale will be determined by the number of years. It will be set for him like the daily wages of a hired worker.

51If many years are still left, he must pay his redemption price in proportion to them based on his purchase price.

52If only a few years remain until the Year of Jubilee, he will calculate and pay the price of his redemption in proportion to his remaining years.

53He will stay with him like a man hired year by year. A resident alien is not to rule over him harshly in your sight.

54If he is not redeemed in any of these ways, he and his children are to be released at the Year of Jubilee.