Set up a bill rate override to recognise revenue at different rate for a specific expenditure type

How can we set up a bill rate override to recognise revenue at a different rate for a specific expenditure type?

Summary:

We would like to generate revenue at 100% for one expenditure type, but with all other costs at the contribution rate on the contract.

Scenario - A project has incurred cost for £100k partnership and will claim £100k back, the rest of the costs are claimed back at 80% in line with the contribution rate on the contract.