@Kampfarsch Not the first isekai where the money system is confusing. It's kind of strange coming from a Japanese author, though, since Japan was an isolated island in the past. In real medieval Europe, the money system actually was confusing, though partially dependent on where you lived. The various countries had their own coins, but the coins from trading partners were also in wide circulation. The worth of a coin was based on its total silver content (or gold if you were doing major business; not something concerning commeners), but coins from different countries were of different sizes, thus having different amounts of silver. A commoner could easily be fooled out of their wealth by a crafty merchant if they weren't careful. Naturally all the currencies from different countries had their own names.
In any case, Japanese isekai authors often invent some weird currency that's worth as little as the modern yen for the sake of Japanese readers. It makes zero sense in a historical context, but isekai authors aren't known for their quality of writing, quite the opposite.