Midrash su Levitico 25:51

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."

6) — whence we derive that one who was sold to a gentile has the upper hand (in the reckoning). Whence is it derived that when he is sold to a Jew, when he is redeemed he has the upper hand? From "hired man" (here) - "hired man" (Devarim 15:18, with respect to one who is sold by beth-din) — a gezeirah shavah (identity) — Just as with "hired man" mentioned in respect to (his being sold to) a gentile, he (the servant) has the upper hand (in his redemption), so, with "hired man" mentioned in respect to (his being sold to) a Jew, he (the servant) has the upper hand in his redemption.