Customer Billing but keeping inventory to deliver to their customers

Dear Experts:

We have the next scenario:

1. We sell material to one Customer (this material is only for them, we don't sell this item to anyone else)
2. Customer ask us to keep inventory in our storage.
3. Customer ask for invoice to keep control of "how much material" they have with us. (They pay this material, even if the material is still in our facilities).
4. Later, we need to ship the material (at this moment, really Customer ´s inventory) from our storage location to their customers.

Right know, what we are doing is bill to customer with a service material (LEIS) for the first invoice, and when we ship to their customer, we create another invoice with the FERT material, and then we create a credit note to cancel the first invoice....

But we don't know how much of my stock is really ours, and how much is already billed to our customer.... Also, customer isn't very happy with the credit notes....

We don't know the correct manner of doing this in SAP, and I can't find a similar scenario neither in SCN or in SAP Help....

Any help would be apreciated!...