Three is the magic number
Now: three budgeting types. Fixed price, Time and expenses, Non-billable.
Fixed price
So, the Fixed price budget type allows you to do just that. Enter the budget when you start a project, and you’re set. In the end, this amount is the one that will be invoiced, as well.
We know many clients want to check the progress made on a project. Likewise, you might need to know how many minutes, hours, or days have been invested in various tasks. And you can still do that! The tracking of time and expenses is possible, but it won’t affect the above mentioned predefined budget. What will be affected are the costs and the spent amount of the budget.
Time and expenses
Simply track the time directly on tasks when working on them, make sure you marked them as billable (or set all records to be billable by default on a project/task level), and then charge your clients for your work.