Bill estimated time not actuals (fixed cost)

We have been running with intervals for three months and what i want to achieve is to be able to bill by task estimate for projects. If the task that need to be completed was 10 hours and the resource completed it in 5 hours, currently the system displays revenue against the 5 hours only. I want it to run of the estimate time as this is what the client has agreed to pay (fixed cost).

if i look now i will have to manually pull the time time down to check this. lickewise if the time estimate was 10 hours, and the actuals was 15, i only want to bill the client for 10.