Rashi on Leviticus 27:18/רש"י על ויקרא כ״ז:י״ח

1ואם אחר היבל יקדיש BUT IF HE SANCTIFY [HIS FIELD] AFTER (i. e. some years after) THE JUBILEE, — and similarly, of course, if he dedicated it from the year of (i. e. immediately after) the Jubilee, — and in either case it remained for a time in the possession of the treasurer and then this man comes to redeem it some years after the Jubilee,

2וחשב לו הכהן את הכסף על פי השנים הנותרת THEN THE PRIEST SHALL RECKON FOR HIM THE MONEY ACCORDING TO THE YEARS THAT ARE LEFT — it must be according to a reckoning. How is it to be done? You see, it (Scripture) has fixed the value of forty-nine years at fifty shekels, that is, one shekel for each year and one additional shekel for all of them (for all the years together). Now a shekel is forty-eight pundion; that gives one selah (shekel) and one pundion for each year, except that there is one pundion short for all the years (reckoning the price per year as the Torah prescribes, he pays forty-nine shekels and forty-nine pundion, equal to fifty shekels and one pundion whereas really he should pay fifty shekels, which is one pundion less than the calculation based upon the yearly rate. Our Rabbis said that that pundion is the commission for the money-changing (Bekhorot 50a), and therefore if he comes to redeem the field some years after the Jubilee, he must give a selah and a pundion for each year of those years that remain until the Jubilee year (Sifra, Bechukotai, Chapter 10 10; Arakhin 24b).

3ונגרע מערכך AND THERE SHALL BE ABATED FROM THE VALUATION the number of years that have elapsed from the Jubilee year until the year when the redemption takes place.