Can your corner store credit be used to pay taxes? How long will your corner store survive if the IRS found out you were processing transactions in your own currency?
But, at least in the US, before the national bank acts in the 1800's, various taxing entities specified which bank notes (or other assets) they'd accept and not accept. It was basically whichever bank notes circulated the most with no/little discount, some gold/silver coins, some government bonds. Ie acceptance by a taxing entity isn't really a clear line on "is/is not" money.
Trading receivables has been going on forever, happens today, is totally legal. The corner store could sell the receivable, and many businesses to sell their receivables.
Offering someone credit in USD isn't making your own currency. The closest widespread version of that are community currencies[0], which the US doesn't seem to particularly care about -- I'm guessing because they're generally pegged to the dollar and promote local economies.
[0] https://en.wikipedia.org/wiki/List_of_community_currencies_i...