The problem of counterfeit currency and debasement of metals used in coinage has existed as long as money has existed.
Gold as a form of direct currency became unusable once the population using the gold surpassed the number reasonable to divide up the gold into a reasonable size coin. Silver, somewhat more in physical supply substituted, but of course silver couldn't be distributed around either and so coinage in the US started using nickel instead.
For anyone hoarding gold coins or silver slips as a projected means of future tender, until the population drops off seriously, it just won't work for buying food.
Paper Dollars actually at the moment would hold the most security. To combat the Counterfeiting problem, the US MInt has for years pursued ever more intricate methods of making this piece of paper difficult to counterfeit. Paper dollars also represent only a small fraction of dollars in the money supply, so even if the rest of the system goes blooey, for a while at least paper dollars might function to buy you something.
Projecting into the future, I would agree that any new currency system would first be based on direct barter, only later to be followed by some sort of private Promisory Notes that might be honored locally as tender once some trust was reestablished. This kind of system could only work very locally however, which is one of the reasons I favor small community sizes no larger than around 10,000 human souls. For trade on the grand scale, its direct barter again. We ship a tanker full of oil down to WA, at the dock its traded directly for a container ship full of food. Forget the Letters of Credit.
Of course, there can be corruption and theft at the docks, as there has always been. Always wise to accompany your Cargo Ship with a Frigate and some Cannon to bombard the port if they try to cheat you
Or an Aircraft Carrier with some Supersonic Jets, long as you can keep them running.
Reverse Engineer