Leviticus 25:48-52

48then after he is sold he shall have the right of redemption. One of his relatives may redeem him:

49either his uncle or his uncle’s son may redeem him, or one of his blood relatives from his family may redeem him; or if he prospers, he may redeem himself.

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.