If I buy a radio at Walmart, is that going to make you go to Walmart and buy a radio using modern currency?
No, but if I will certainly be confused and think that now using fiat currency is permissible. And if using fiat currency is indeed a sin in God’s eyes you’ve just made me think that something that is a sin (ie. using fiat currency) is permissible. That’s the real issue.
Also, the type of issues Paul dealt with were a lot more grounded than what you are talking about here. People were concerned with either not eating meat offered to idols (or meat that might be offered to idols) or else kosher food laws. There was at least some basis for their concern in the Old Testament.
Dishonest weights are also an issue talked about in the Old Testament. So I do have reasons for concerns because fiat currency is not only based on dishonesty, it is outright fictitious wealth packed with lies. Liars will have a part in the lake of fire, why shouldn’t I be concerned about using a currency that will turn me into a potential liar?
The price of these commodities goes up and down against each other just like it does against gold or fiat currency.
Which is why I said wheats and barleys should be weighed, not assigned prices.
This why did John the Baptist give this advice?
Luke 3
12 Then came also publicans to be baptized, and said unto him, Master, what shall we do?
13 And he said unto them, Exact no more than that which is appointed you.
Because it is not practical or even possible for all publicans to abandon their jobs like Matthew did. But John did tell them to do their best to minimise the evil by not asking for extra tax money beyond what they were asked to collect by authorities.
The same principle should be applied to fiat currency. If you are able to abandon it like Matthew did his tax occupation, then do it. If not, minimise the evil as much as you can. That is why the OP suggested prepaying expenses and buying necessities in advance. Yet people accuse him of doing evil when he is trying to get rid of what he believes is evil.