The last 6 months, I worked for a person (1 month at their office, 5 months remote, living not far). During the last month (March) another developer joined the project, the management became hard, because this boss was motivated (which is good) but his will to participate in development added noise, because he imposed a lot of technical directions, constraints, not always justified, despite the fact that he is more a designer than a programmer (without offense, I always respected him, and learned to understand him). In the beginning of April, we had a video call, and with the accumulated frustration, stress, we both decided to stop collaboration, just after in chat, I sent a message where I say (without reflection, and in the emotion of the situation) that I just want a compensation of 500€ for all this work. He said ok, I add that I just would prefer the laptop (value: 200€ (cheap one)) that I used to work with in their office, and that they don't use, he agreed too, I took it. But I acted without reflection, in the emotion of this project I really like, and the disappointment of this person's attitude.
Many chat messages prove an agreement for a higher bill. So a few days later, (after he sent me other emails), I decided to make a more important bill, like I had planned to do for months with mutual agreement, (the total of the bill is just 1 month of salary, so ridiculously low, that doesn't compensate my expenses). I emailed him, politely for the bill, and for the first time, he doesn't answer.
I compiled all I had on this project, there are many emails (direct ones, and gitter/drive/github notifications, fortunately), chat messages, and there's also the git codebase aside. He tried to erase those traces (he deleted the main gitter room, and removed my access to the files I contributed to make.
Thanks for the advices (the other developer got paid in advance so he won't have this problem).
He still has 1 week to pay, so I hope he will
edit: There are proofs from the chat rooms (private gitter rooms data can still be fetched with their api and can't be deleted by him), and quotes like: "I'd have preferred to give you those 1200€" (talking about payment to the other dev) and other agreements to pay more than 800€/month (btw I'm just asking 2k€ in the final bill, which isn't high for such a contribution). I had seen he was a bit impulsive, but I was still focused on the work, and except my 2 last stupid messages on the chat where I lost a bit my reason, the other thousands of messages show a global agreement and satisfaction, of course with small disagreements, but usually after explanations we could agree.