You're being more literal than I was. My point was that "a debt" is a broader concept than the GP comment acknowledges. A debt is incurred any time you propose or agree to buy something. And legal tender is the way you settle it.
But also, I think that most people would not consider any debt to be incurred for transactions where the payment and receipt of goods is done at the same time, like in a grocery store.
If there was a posted notice that no cash is accepted it's unlikely you'll get a criminal charge, but you can get civilly sued. Most places will just accept the cash then put up a picture saying "If this asshole shows up again, trespass him"
No, eating food & then paying is a debt. After the services have been rendered. If seller can pull back the items, never provided the service, no debt.