Midrash su Levitico 25:27

3) (Vayikra 25:27) ("And he shall reckon the years of its sale and he shall return the balance to the man to whom he sold it, and he shall return to his holding.") "And he shall reckon the years of its sale": He reckons years and not months. Whence is it derived that if he (the seller) wishes to convert months to years he may do so? From "And he shall reckon." "and he shall return the balance to the man to whom he sold it": Whence is it derived that if he sold it to the first for one hundred, and the first sold it to the second for two hundred, that he reckons only with the first? From "to (the man) to whom he sold it." If he sold it to the first for two hundred, and the first sold it to the second for one hundred, whence is it derived that he reckons only with the last? From "to the man" (who is holding it). These are the words of Rebbi.

4) R. Dostai b. Yehudah says: Whence is it derived that if he sold it for one hundred a year and it appreciated in value to two hundred a year, it is assessed at only one hundred? From ("and he [the seller] shall return) the balance" — in his hand (from the original transaction, i.e., one hundred per year). If he sold it for two hundred a year and it depreciated in value to one hundred a year, whence is it derived that it is assessed at only one hundred a year? From "and he shall return the balance" — the balance remaining in the land (at present). "and he shall return to his holding": I (the L–rd) have spoken only of one who will return to his holding, and not of one who sells to another (for profit).