Scrolling through Instagram one day in March, Ashley Light froze on a post showing a map of Texas with her ZIP code highlighted.

Then a local reporter showed up. A few days later her bank called and a letter from the state arrived, all confirming what she had seen on social media. Her borderland neighborhood in El Paso was being targeted by the Trump administration.

Just a few months earlier, after her father died, Ms. Light had taken over her family’s small money services business, the Valuta Corporation, which offers check cashing and currency exchange. Now her company — and all others like it in 30 ZIP codes scattered across Texas and California — was suddenly required to report any transaction of $200 or more, along with personal identifying information about the customer, to the government. For decades, the reporting threshold had been set at $10,000.

The Treasury Department billed the enhanced scrutiny of already highly regulated businesses as a crucial element of President Trump’s strategy to kneecap Mexican cartels and stem the flow of illicit drugs and money across the southern border.

But owners caught in the crackdown said they found the new regulations crushing. And the theory that they may be blind participants in violent criminals’ schemes to conceal dirty money bore little resemblance to the border communities and working-class customers they knew.