Sounds like you got scammed. How could he just change the amount for the ride? Isn't there a meter that's visible to both of you that shows what's due? If he insists that you have to pay more than what the meter says, tell him to get bent.
The irony is that I actually was about to take an über x quoted at $70, but I decided on a cab last minute (wish I hadnt).
You have been scammed, proper course of action would be calling police on the spot (unless you are black).