Leviticus 25:51

If [there be] yet many years behind
To the year of jubilee, and more than he had served: according unto them he shall give again the price of his redemption,
out of the money that he was bought for;
suppose, for instance, when a man sold himself, there were twenty years to the year of jubilee, and he sold himself for twenty pieces of money, gold or silver, be the value what it will; and when he comes to treat with his master about his redemption, or a relation for him, and he has served just as many years as there are to the year of jubilee, ten years, then his master must be paid for the price of his redemption ten pieces of money; but if he has served but five years, and there are fifteen to come, he must give him fifteen pieces; and so in proportion, be the years more or fewer, as follows.