Tracking sales by distributors to end customers

My client has a process of 'Price on Delivery'. The way it works is that the client has an agreement with the end customer say selling the product at $20.00 for the year through a distributor. This price is fixed for the end customer for the year, however during the year they may be selling to distributors in between say at $25.00 and then the distributor will have to sell it to the end customer at $20.00. Like this there may be multiple variance in the prices and then then the distribitor raises a claim for $5.00 and so on.