How do we invoice for Contingent Fee (%-based fees) in QBO?

We are a consultancy business that operates two sets of products; (i) hourly-rate advisory, (ii) a contingency-based fee with a fixed percentage of tax reclaims. We use QBO Advanced.

Example

We reclaim £10,000.00 in overpaid tax and keep 10% as a fee.

Therefore, we invoice the client for £1,000.00

Question

Is there a correct way to structure the Estimate/Invoice for the contingency-based fee products? I am struggling with the imagination on how to make this work and the future permutations for this (for example, the Estimate is at 10,000.00 but we only get 8000.00 back in reclaims.

Any help or ideas would be greatly appreciated.