Know Before You Go
This situation is almost always preventable. A verbal agreement is nearly worthless in the music industry. Before you ever load in, you need a written contract or at minimum a confirmed email thread that spells out: the fee, when it's paid (before or after), and what happens if they cancel.
In The Moment
Stay calm and professional. Don't blow up at the booker — you may need to work in that town again. Calmly reference your agreement: "Our agreement was $X, as we confirmed on [date]. I need that amount tonight before we leave." If they push back, stay firm. Being polite but immovable is more powerful than being aggressive.
If They Still Won't Pay
Document everything. Screenshot messages, save emails, note down who said what and when. For amounts under your local small claims court limit (often $5,000–$10,000), you can pursue this yourself without a lawyer. File a claim, attach your evidence, and let the legal process do the work. Venues hate the hassle and bad reputation that comes from being taken to court — often they'll settle before you even get a hearing date.
Nuclear Option
Post a factual, non-defamatory account on musician forums like GigSalad community boards or local Facebook musician groups. Don't rage — just state the facts. Other musicians will thank you and the venue will feel the pressure.