one good turn deserves another

One good turn deserves another.
(A good deed should be repaid with another good deed.)
Долг платежом красен.

Example 1: If he does you a favor, you should do him a favor. One good turn deserves another. (thefreedictionary.com)

Example 2: Glad to help you out. One good turn deserves another. (thefreedictionary.com)