I haven’t looked at it yet, but maybe Keys from the Golden Vault has something you could use for inspiration? Seems like the adventures therein are meant to be ‘heist-y.’

In general, keep the following in mind when making an adventure:

• WHAT? What is the premise of the adventure? In this case, it the basis for your post: “Counterfeit currency is being circulated through the local economy.”

• WHO? Who is/are the primary actors in the situation? In this case, WHO is circulating the currency? Are they in a traditional position of power (government, community leader, faith leader) or are they a small fry or underdog (peasant, small organization, charlatan)?

• WHERE? The setting and context matters. In this case, is “the local economy” a kingdom, a single city, a small town…? This can define the scope of your adventure and give context as to why the issue needs adventurers to resolve it. A city’s treasury might not have the manpower to mount an investigation, a village might not understand why it’s a problem, or maybe even largely not care (“it’s only a problem for the merchants”), a kingdom might be dealing with a different or related financial crisis.

• WHY? What precipitated this issue? Is someone just trying to get rich? Is it part of a greater political maneuver to frame or otherwise disgrace someone? Is this setup being used to fund some greater scheme (set up a longer plot)?

• HOW? How is the plot being accomplished by the big bad? You don’t generally need to go into intricate detail. The rule is generally “go two layers deep” to make it seem like everything is intricately thought out. “The antagonist is actively spending fake currency around the city to put it into circulation. Why can’t the merchants tell it’s fake? The core of the coins is an amalgam whose weight matches gold.” “The antagonist is replacing coins in the treasury with fake coins. Why doesn’t anyone know this is happening? The antagonist found a secret passage on an ancient map.” Etc