LOL wow, first time I've ever heard someone make that much money. You've hit what's called an "integer overflow", basically where a number becomes more than the maximum number that can be stored in memory (about 2 billion) and so it wraps around and becomes the minimum number (about -2 billion).
You can probably fix it by spending enough money so that you go below the minimum and wrap back around to the maximum. (Maybe buy a bunch of FTL fuel, which you can do while in debt.)
I'll fix this for the next update. 😛
