Last donation dictates the charity that gets the money and resets the timer
And the minimum amount must be >= last donation.
Edit: Wait, are we reinventing poker?
What if you donate on behalf of a charity and which ever charity has most donations gets the entire pot?
Sounds closer to reinventing the penny auction (where did all those sites go?)
For a charity, you could choose a good cause that upsets assholes. So they have to donate money to the charity to prevent it from getting the money that was donated.