За добро добром и платят

If you help people, they will repay in the same way; if somebody does you a favo(u)r, repay in the same way. See Добро век не забудется (Д), Добро добро покрывает (Д), Живи для людей, поживут и люди для тебя (Ж), Кинь в окошко крошки, в дверь придет лепешка (K), На добрый привет и добрый ответ (H), Услуга за услугу (У), Хорошее дело два века живет (X)

Cf: An act of kindness is well repaid (Am.). Do well and have well (Br.). Do well, have well (Am). Give and take (Br.). Give and you shall receive (Am.). A good deed is never forgotten (Am.). A good deed is never lost (Br.). The good you do to others will always come back to you (Am.). The hand that gives, gathers (Am., Br.). Kindness always begets kindness (Am.). A kindness is never lost (wasted) (Am.). One good deed deserves another (Am.). One good turn deserves another (Am., Br.). One kindness deserves another (is the price of another) (Am.). One never loses by doing a good turn (Am., Br.). Repay kindness with kindness (Am.). Scatter with one hand, gather with two (Am., Br.)