If I do not have any claim against a supermarket (and until today I had not), then I logically can not reduce their claim for groceries from my non-existing claim!

It's funny how you pretend to be so smart or so dumb depending on how it suits you. I hope actmyname doesn't mind if I crayon this for you.
Plumber unclogged a toilet in the supermarket therefore he has a claim against the supermarket. You pay the plumber and you get the groceries. I wonder why it doesn't work like that in the real world - sounds like a fantastic system based on your last few hundred posts.