you owe me 50 cents
If a friend says to you 'you owe me 50 cents' it means you have had sexual intercouse with someone that they have had sex with in the past.
Context:
Bill: I had sex with Sarah on Tuesday
Sam: Oh mate, you owe me 50 cents.
Bill: I had sex with Sarah on Tuesday
Sam: Oh mate, you owe me 50 cents.