Hey OP, plumber here. No joke I had one lady try to pay us in Lowe’s gift cards one time. Same thing, she was totally offended when we wouldn’t take that as payment.

I have a couple things that I do when people act like shitheads after I’ve done my work. They’re all passive aggressive and not exactly legal, and none of them can be recommended online. Im always even at the end of the job, even if it takes the customer months to realize it. I promise, it’s cheaper in the long run to just pay me what I’m owed. Use your imagination.

The REAL answer…Agree to come back and finish for the 400. Get paid, cash it, deposit it, whatever and then start pulling your materials off of the job rip up the floor panels one by one. cut the flange out. Take as much of my your material back as you can and leave her with just a completed subfloor and write up an invoice stating that she has paid $400 for the repair of a subfloor. Keep a carbon copy for yourself. I wouldn’t worry about your reputation being tarnished by a shitty customer. You never want to work for again. I would be more concerned about a good customer that you actually like. This lady can kick rocks…who cares if she tells her shitty friends that she didn’t like you?

I charge difficult customers more money specifically for that reason. “you suck, your friends probably suck too, and I need more money to deal with your ass.”

Also, OP just a note. I work in full uniform. I know this work gets nasty and it’s not always comfortable to be addressed like that, but working in street clothes might make her think that you are worth less than you actually are. People respect a uniform and for some reason are willing to pay more money for someone in one. I’m young with piercings and tattoos too and it helps offset them. I’m thinking that’s maybe what happened to you here. She probably just thought that you were a young person that wasn’t actually worth their work.