Leviticus 25:50-52

50And he shall reckon with him that bought him from the year that he was sold to him unto the year of jubilee; and the price of his sale shall be according to the number of the years, according to the days of a hired servant shall he be with him.

51If there are yet many years, according unto them shall he return his redemption [money] out of the money that he was bought for;

52and if there remain but few years unto the year of jubilee, then he shall reckon with him; according unto his [remaining] years [of service] shall he give him back his redemption [money].