In reading this thread, I think you have larger problems than just if your partner is overcharging you $200
You wrote that the house was built in the 1800's and needed major, "never ending" work, so it's surprising to see that you've only had to spend $8,000.00 so far and are already over budget. What is the extent of the work and what was your budget?
Also, if he is "acting as the GC", hiring people and overseeing their work, then that is "work". If he's paying these guys cash, you still need receipts, etc. Are you not going to file 1099's for them or deduct these expenses from your taxes? Do they have insurance, etc. Did you procure the proper building permits? What if someone gets injured on your jobsite? Do you have that covered?
I'm sorry but it doesn't seem that you are properly set-up to be doing a project like this and you are exposing yourself to a lot of risk. As a practical matter, to control what he spends, why don't you pay the guys directly?