@Talismaster The coins should per definition not be possible to be counterfeit. If the country is backing the coinage, then the coins are legit. However, if the king decides to proclaim that the money minted on the order of the aide is counterfeit, they will become as such as the backing for the coinage is retracted. As for why I mentioned you? Because what you said is exactly right. What happened is just inflation, and any mention of counterfeiting in the manga is just irrelevant to their value.
Feels like the way the manga (mis-)used "counterfeit" is to describe coins that had lower percentage of precious metals in them. Countries do that at times, and back when the amount of gold and silver mattered, this is essentially what showed the economical situation of the country. If a country had coinage with more gold in it, it was more *likely* to have a stronger economy (this is running on the assumption that the prices of the commodities still costed the same number/weight of coins, and they didnt just artificially deflate the economy by minting fewer coins. Essentially the actual indicator is that the country owns more gold in active circulation than others, so gold is worth less within their borders). Iirc there was an example in Spice and Wolf of a country devaluing their coinage (and trying to be sneaky about it), and similar things pop up as by-the-by's in other novels all the time.