No I'm not changing the terms you agreed to. If you're only just realising what you agreed to 3 months later that's your problem.
The bet stands as you agreed to, $100 Trump loses to the charity of my choice.
You're right when I made the bet I knew what I was doing, clearly you didn't. I still haven't decided on the charity if I win the bet, maybe I'll make it the same as if I lose. Maybe I won't. Your actions so far in trying to change the bet twice, that you agreed, to hardly sways me to choose the same charity.
