Currency does not get used up. It remains somewhere, no matter how often it is traded. Its only use is as a repository of value that can be more easily traded and exchanged for things that can be used or used up.
NAILED it! Yes, I can trade you sage, salt, and red pepper for sausage, and you can trade me a gun repair for a dozen eggs- but that is barter.
Currency- look at the definition of fungible. It serves as a SYMBOL of value that is more portable than the items that it buys or sells. It is durable, is not consumed, and you cannot easily or readily make a fake version (reason that counterfeiting used to be a capital crime)
Fiat currency (it has value only because we SAY it has value) may go away, but the other things we USE as currency (gold, silver, copper, gems) will remain in use as such. No, ya can't eat silver dollars. You also cannot put 5 barrels of flour in your pocket and walk around with it.
In the discussions of TEOTWAWKI, I tend to look at the world's WORST piece of crap cultures- and yes, there IS still a culture, even at Stone Age levels. Afghanistan, back in the Kush. Africa, away from the cities. Serbia. The steppes of Siberia. What gets used as currency? Rare items that do not rust, rot, or get consumed. In other words, precious metals and gems.
Yes, I know the saying in Spanish- "Plata o Plomo" (Silver or lead) Yes, you can be the bandit, and take what you want by force. THAT has usually been the forming point for societies- either the bandit BECOMES the governing body (Scottish clans) or people join together to squash the bandit.
Look at the small towns when the American West was being settled. What were the hallmarks of a decent town? Merchants (and bars)- sure. A post office and telegraph office- yep- communication. School- maybe. But the way you knew that you had a town and not a collection of buildings- you had a BANK. A place to store and exchange tokens of worth.