Monty Python's Flying Circus, Series 3

Mr. Vibrating:
I'm very sorry, but I'm not allowed to argue unless you've paid.

Man:
Aha! If I didn't pay, then why are you arguing? Got you!

Mr. Vibrating:
No you haven't.

Man:
Yes I have. If you're still arguing, I must have paid.

Mr. Vibrating:
Not necessarily. I could be arguing in my spare time.

Man:
Oh, I've had enough of this!

Mr. Vibrating:
No, you haven't.

Man:
Oh, shut up!