From Longman Dictionary of Contemporary Englishfair’s fairfair’s fairused when you think it is fair that someone should do something, especially because of something that has happened earlier Come on, fair’s fair – I paid last time so it’s your turn. → fair
Examples from the Corpus
fair’s fair• Come on, fair's fair - I paid last time, so it's your turn.• Come on, fair's fair. It's your turn to mind the kids. 