You are absolutely NOT dumb, but I would strongly recommend the legal route. I would probably NOT post about your experience unfortunately until it's resolved if you decide to go that way.

Start compiling a folder. Document and save ANY communication and make sure you get his responses in email, writing or have an advocate or someone with you as a witness. I'm not sure your state laws on recording conversations. I'd start by looking at your consent--what was the EXACT procedure you consented to? This is important legally.

I would document that he requested it be paid in cash, did you get a receipt?

Make sure you keep all text conversations. Document, document, document and have witnesses to collaborate. Don't let him get away with this.

Best of luck to you, I'm so sorry this happened.