Midrasch zu Wajikra 25:52

5) (Vayikra 25:51) ("If there are yet many in the years, according to them shall he return his redemption from the money of his acquisition.") And whence is it derived that he goes out by deduction of money (i.e., by deducting the value of the years that he worked from that of the years until Yovel and giving that as redemption)? From "If there are yet many in the years, according to them shall he return his redemption from the money of his acquisition." Whence is it derived that if he were sold to him for one hundred a year and he increased in value and is now worth two hundred a year — whence is it derived that (for purposes of redemption) he is reckoned at one hundred a year? From "he shall return his redemption from the money of his acquisition." (Vayikra 25:52) ("And if few are left in the years until the Yovel year, then he shall reckon with him. According to his years shall he return his redemption.") If he were sold to him for two hundred a year and he is now worth only one hundred a year, whence is it derived that he is reckoned at one hundred a year? From "According to his years shall he return his redemption."