Leviticus 25:50-52

Counting only the years from the time of his selling unto the year of the jubilee: and counting the money that he was sold for, according to the number of the years and the reckoning of a hired servant.

If there be many years that remain until the jubilee, according to them shall he also repay the price.

If few, he shall make the reckoning with him according to the number of the years: and shall repay to the buyer of what remaineth of the years.