That Time I Got Reincarnated as a Slime: Tensei shitara Slime datta Ken

[Kraco]

There's no merchant that travels to Country X expecting the customers to pay in Country Y's currency, while all the time knowing full well Country X doesn't officially use that particular currency. Maybe that currency would exist in that country to some degree, maybe it wouldn't, but why would they specifically count on it? The only exception is if it was beforehand agreed that they would be paid in that currency. Like oil has been typically traded in dollars in our world. However, if that was the case, why did this whole incident happen? Rimuru would have known beforehand they need to have a lot of dwarven coin, so he would have arranged to have it ready and waiting. From this we know the foreign merchants most definitely were not promised dwarven coins when they were invited in.

Sure, technically those goons probably were merchants in their original countries, but they ceased to be merchants and became disposaple agents the moment they agreed to work for the mafia. They probably were promised some kind of big reward for it, in addition to any normal profit they would make by selling their goods. Otherwise they would have accepted any sort of valuable material as payment, knowing they could use it themselves or convert it into dwarven coins by reselling it later.

[Buffalobiian]

Sure, technically those goons probably were merchants in their original countries, but they ceased to be merchants and became disposaple agents the moment they agreed to work for the mafia. They probably were promised some kind of big reward for it, in addition to any normal profit they would make by selling their goods. Otherwise they would have accepted any sort of valuable material as payment, knowing they could use it themselves or convert it into dwarven coins by reselling it later.

"First of all, Demon Lord Clayman's assets can't be used as currency."
"They are not the currently globally-accepted currency issued by the Dwarven Kingdom."
"And stellar gold coins are worth so much that they can't be used for individual purchases."
...
"By the time I realised it, it was too late."
...
"The big store owners I'm acquainted with may be a bit more accommodating, but the small retail traders are demanding payment in cash."
"According to the rules set by the council, monetary payments must be made using coins minted in the Dwarven Kingdom."
"By the standards of the Western Provinces, the traders' demands are legitimate."
...
"Then what if we paid them in raw gold plus a little extra..."
"The traders who are wise to their own interests should agree to that... But I am vehemently opposed to it!"
...
"They'll remember us as a country that'll respond to unreasonable demands by suffering losses in the name of saving face!"
...(several episodes later)...
"I'm sure you're aware that Dwarven gold is the only currency that merchants trust."
...
"Well, as long as they're obeying international law, we won't complain."




The above is extracted from the relevant episode.




-Mjöllmile called this unreasonable towards the end of the conversation, but at the beginning he actually made it out to be quite reasonable.
-Mjöllmile said that the small merchants should be fine with gold, but they shot this down because he didn't want to set a precedent. There wasn't any actual evidence around the small merchants being pawns. It all sounds like they're here to do business, and when someone cast doubt on Rimuru's country's legitimacy, they demanded payment in the standard that they requested.
-Mjöllmile even said that the big store owners he's acquainted with might be more lenient, which implies that even those who weren't part of the setup would consider Dwarven coins the standard.

What IS quite weird is that this festival is organised as if people can eat for free or something, and Rimuru foots the bill afterwards (rather than citizens paying for their own goods). For something like this to occur, you would have expected payment methods to have been pre-organised and agreed upon. For there to be lackthereof suggests poor writing or inexperience by all trading parties (unlikely). You could chalk it up to gross assumptions (that anyone in the world would know that merchants accept only Dwarven gold - and any deal going forward implies this), but it'd still be something you'd bring up before making a deal.