Fair’s fair
Definition 1 of 1
Pro Tip 1/3Expect Reciprocity
Used when someone should return a favor or give something back in exchange for what they've received. 
You gave me a ride last time, fair’s fair, so it's my turn now.
Pro Tip 2/3Colloquial Use
It's common in casual speech among friends or in informal settings, not in formal writing or speeches. 
We split the bill last time; fair’s fair, you pay this time.
Pro Tip 3/3Agreement on Fairness
Expresses a consensus that something is just or is the right thing to do, without arguments. 
Each player gets an equal turn. Fair’s fair.