hypothetically if folks in Argentina start to use dollars in their banks and only touch physical dollars (as the interface between US and Argentinian money circulation) they are still using real dollars without touching US accounts.

their banks can form their own interbank exchange, they can even make a central bank that does "open market operations" (can buy and sell things on this exchange from pure air - like the NY Fed's trading desk which is basically the executive manifestation of the FOMC), but of course it'll have a mighty hard time to get price stability. not to mention that the US wouldn't be happy