You would have to hope that after you give them your money, they don't kill you to eliminate the witness, or just for fun.
Yes, that part is a factor.
In that situation, I would act very nervous (which won't be that much of an act) and toss my wallet to them, but over their head. Their eyes will follow the money, at which point I would do my Jesse Owens impression.
When I play poker, I carry a fake wallet. Well, it's a real wallet, but it is full of junk, expired credit cards, library printer cards with no more money, etc, and three dollars cash.
I keep my session bankroll in a zippered pocket where the watch pocket of jeans is.