Leviticus 25:27, 51-52

let him calculate the years since he sold it and pay back the balance to the man to whom he sold it, and then return to his property.

If there are still many years left, he shall pay proportionately for his redemption some of his sale price. If there remain but a few years until the year of jubilee, he shall calculate and pay for his redemption in proportion to his years of service.