Money...Money...Money
Prior to modern states and the development of industry on a mass scale, as we experience today, most activities were artisanal, local and non-standardised. This applies to the minting of money. Standardised money (including, interestingly enough, paper money) only came with empires, such as was and is China.
What this means is that local kings and nobles would create their own currencies for use in their territories. The result is that if you went from one place to the next the money from where you came from would be viewed with suspicion if not downright hostility. The problem wouldn’t be acute between places that traded with each other since a degree of mutual acceptance was inevitable. There would be an exchange rate between the two. Two gloches for a minet, or whatever the local spcie was called.
A key function of authority, and we see this in the first states that evolved, was to try and standardise the currency in so far as possible to ease trade frictions and overcome the rejection issue of foreign pieces.
Another way, was to use a common unit of value. This would be precious metals. The usual way to do this was to divide the types of coin by value. Value here being dictated by whether they were base or precious metal. The key ingredient here was the weight of precious metal per coin. Hence, the exchange rate was based on the amount of gold in the respective coins. Equally, since the coins were made of gold, they could be accepted as equivalent to specie. It mean assaying the coins and wasn’t above fraudulent practice. Hence, any adventurer who wished to use their foreign coin might be subject to all sorts of issues and complications.
The above provides a possible plot point and even some opportunity for some humour and so on.
A final thought is that the need for standardisation meant that reputable kings and their mints would be readily accepted, whereas the ‘unknown’ coins would be shunned.
I think in fantasy stories, issues with money (having/not having and so on) are not really given much atterntion. Perhaps it is time to change that.