Midrasch zu Wajikra 25:53

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.

7) (Vayikra 25:53) ("As a hired man, year by year, shall he be with him; he shall not rule over him oppressively before your eyes.") — What he (the master) eats, he (the servant) eats; what he drinks, he drinks.

8) "He shall not rule over him oppressively": I might think that one should enter his (the gentile's) house to discover what is being done to him (the servant); it is, therefore, written "before your eyes" — You are commanded only in respect to what you see before your eyes.