Over the summer, in addition to working as an employee of a software company part-time and studying part-time I took on a contract project to make extra money. I gave them an estimate and we agreed to an hourly rate, but I never got anything in writing.
The project involved specialty hardware which had to be ordered, so it was slow to start. At the end of the summer I lost my first job (on very amicable terms, project finished) and went in to present a prototype of the contract project expecting payment in full for the hours I'd submitted. I'd also sent in some receipts for a few hardware items I had to buy myself to get things working.
The owner, who'd been in the hospital recently according to email correspondence, only gave me 1/3 of the money I was owed at this point and avoided answering any questions about when I'd be getting the full amount. Previously, I had been doing a lot of phone consulting, emails, and running around without even logging the hours.
This caused me a lot of stress, but I was able to find another job. My new job is much more interesting and now I'm back at school full-time. This and being involved with the ACM means I have no time for this old project. I also just got an interview with a big software company (they're flying me in), so I'm really busy. They did send me a bit more money, so they're up to about 1/2 now, but I still have no guarantee about when I will be getting the rest of the money.
The owner wants to know when I will have time to finish the project (about 75% done now), which they just ordered the last hardware component for, but I don't have any time right now. Even if I wanted more money, I could just put in more hours at my new job, and I'd know when a reliable payment would be coming.
After the stress they caused me, especially seeing as how I was going above and beyond by answering all of their emails without keeping track of the time, I'm pretty upset. Since I have no idea when I'd be getting the rest of my money, much less payment for future hours, I have no motivation to continue with the project.
I still have their hardware at my house however and they still owe me some money. What's the most professional way for me quit? How should I go about getting the rest of money seeing as how I didn't get anything in writing? I did email in two invoices, both showing the amounts owed and the payments they made, which were not contested, so I have a little bit of a paper trail.