Are u dumb or are you drunk? The alien didn't assume shit. He predicted with 99.9% of accuracy what you would do. If you try to come out with 1000100 dollars you'll only get 100: THE ALIEN WARNED YOU ABOUT THAT. Why are you still trying to do so?
it still doesn't matter. The boxes are already filled. Your choice is erroneous. One contains 100 bucks, one contains x where x is 0 or 1,000,000. Neither is negative. A+B where B is non negative number is always >= A.
Like my third paragraph said, really it is just the alien being a dick. The order in which he takes action towards being a dick does not make him less of a dick. His dickery aside though, A+B>=A.