...or Your Money Back — Randall Garrett

Jason Howley, an inventor, has arrived somewhere in Neveda from New York City - both in US. Gambling is legal in former, illegal in later. He has come here to gamble - as a way of attracting world's attention to the "psionic" "science".

The plan is to use his curious gadget at a prominent local gambling joint to effect big wins. Idea is to attract attention, get caught by cops, get even more attention during the court trial, & come out scot free because no one can even begin to figure out how the gadget can influence the wheel. In this endeavor, he will enlist support of a competent unnamed narrator & local lawyer who will manage the legal end of drama.

He uses the gadget openly at the roulette wheel in Golden Casino. Quickly converts his $50 into $3,00,000! Attracts attention of the establishment's goons & cops. Gets arrested. And we are treated to good fun during defense. Of course, he gets away scot free - how can the court declare a gadget that works like a good luck charm illegal?