Leviticus 25:50-54

50Then he [or his redeemer] shall calculate with his purchaser from the year when he sold himself to the purchaser to the Year of Jubilee, and the [original] price of his sale shall be adjusted according to the number of years. The time he was with his owner shall be considered as that of a hired man.

51If there are still many years [before the Year of Jubilee], in proportion to them he must refund [to the purchaser] part of the price of his sale for his redemption and release.

52And if only a few years remain until the Year of Jubilee, he shall so calculate it with him. He is to refund the proportionate amount for his release.

53Like a man hired year by year he shall deal with him; he shall not rule over him with harshness in your sight.

54Even if he is not redeemed during these years and under these provisions, then he shall go free in the Year of Jubilee, he and his children with him.