I think you're conflating currency with interfaces to the financial system. The currency is the medium of exchange: the dollar itself. A debit card is merely a tool to transact using dollars.
I'm not conflating things. They're currency. I talked about this with my dad, who was head of the finance department at a college. He had a degree in economics from MIT and an MBA from Hahvahd.
> A debit card is merely a tool to transact using dollars.
And banknotes were just a convenient tool to transact gold that was on deposit in the bank vault.
> A debit card is merely a tool to transact using dollars.
And banknotes were just a convenient tool to transact gold that was on deposit in the bank vault.