First problem I see is that I assume you didn't get anything up front. If they balk at paying a percentage up front then I wouldn't walk, I would run!
Second, don't do anything for nothing! Get the extras in writing with costs.
I don't know how big this job is or what exactly remains from the actual contract and it's cost. I would like to see you complete the contract because then there can be no dispute that you defaulted.
I don't know how you are going to do this now seeing how you p***ed this guy off even though he deserved it. Sometimes it's better to hold your tongue and finish your work and then demand payment.
-Hal
