How to incremental bill

Hello,

I'd like to use PSA to track incremental billing against a fixed price contract. Kind of like a reverse retainer contract. Instead of being paid upfront for the retainer and chipping away at it with billable labor I'd like to create a fixed price contract for a specific value then bill against that contract normally until it hits that fixed price limit. Any labor against the fixed price contract after the limit would be zero. 

Thanks for any ideas/help!