Unit Billing

Need a solution in Projects for the following billing process:

Customer is billed based on Units at an agreed rate.
Each Unit is a combination of hours performed against (sub)tasks.

Example:
Unit A:
- task 1: 6 hours|
- task 2: 8 hours
- task 3: 6 hours
- task 4: 4 hours
There will be multiple Units to be performed.
The number of hours is budgetted as shown.
The actual hours performed could be more or less while the bill amount remains the same as agreed per contract.

Can create a task Unit A with 4 subtasks to register the hours and bill by Events.
However then there is no relation between the hours (cost) and the billing to monitor the margin on each unit performed.