I'll forge a rubber check for you

What you snortingly/contemptuously tell someone who irrelevantly asks to be paid for something that he should let you have for free, such as an item that he'd already discarded. The joke is that said financial instrument would be DOUBLY worthless --- not only would it be a phony check, but the written info on it would be counterfeit, as well. :P

A humorous-natured local woman gigglingly asked me if I'd slip her a few bucks for some old chipped enamel saucepans that I'd salvaged from her curbside trash-pile, so I made a face at her and agreeably responded, "Sure thing --- I'll forge a rubber check for you for five dollars!"