As a disclaimer: I'm working on a dark comedy setting where it's a generic soft fantasy with the aim of parodying the likes of DnD and GoT and LOTR, so things might be a bit whack.
So our cheerful protagonist and his merry band of companions/mercenaries, etc., traipse through some random backwater hamlet on the way back from somewhere more important. Whilst there, they pop by the local vendor who is selling the usual fare: food, drink and Viscount of Dunno's great great great great great grandfather's lost enchanted sword that some kid swimming in a cave lake had found decades ago by accident. This sword is monstrously expensive. For example, if a loaf of bread costs maybe 1 coin, this sword would cost 50000 coins. So our hero promptly dumps 50k coins onto the table (don't ask where the party was storing it) and buys it off the vendor.
As far as I can understand economic theory, a massive influx of money into such a small localized economy could potentially wreck it with inflation (I'm assuming the vendor might use his newfound wealth to go to the village market and spend his heart out, and those village vendors then go to another town market and spend their hearts out...).
But I don't want this to happen. I need the sudden influx of cash into the local economy to.... not crash or raise much of anything. (so when the next party wanders through, they won't be paying hyper-inflated rates for bread).
How can my local economy avoid inflation?