Honestly, it doesn’t really matter, even today. Even though US greenback dollars are the official currency of the United States, there’s no requirement for a state or private individual to conduct business solely in USD. The only entity that is requires to conduct business in USD or USD equivalents is the US government.
For example, when the US Treasury accepted payments from my bank in Italy for tax purposes, an intermediate ForEx broker was involved to convert the Euros to USD after the money left my Poste Italia account and before it hit the US Treasury. The intermediary set the exchange rate and made their money of the delta. I simply instructed that the US government had to receive $XXX.XX and PO and their ForEx sent enough EUR to cover the transaction.
Most of this stuff strips down to whether or not two parties can settle on a means for the values to be exchanged.
Right. Your Euros had to be converted to USD to make the transfer.
I'm just not sure that HEB would be willing (or eventually even be allowed to) convert your 10-round mag of 9mm (traded for that can of beans) into USD - to satisfy their record keeping requirements for financial statements/taxes/etc.
Not trying to split hairs, here. I just have no doubts that the .gov will establish and enforce whatever regulations it needs to maintain control over what we buy and when.
Who knows what's ahead. Maybe there'll be shady intermediaries in the parking lots outside HEBs (with outlandish exchange rates) that will 'buy' your ammo for USD - so HEB can sell those beans to you, in their now required USD, and you can eat.