Special MM business process

Hi Team customer has SAP MM module already but has a business scenario which I am not able to find how to handle this in SAP. Customer usually forecasts certain number of electronic boards which is needed with his supplier. To make this board supplier uses majority of his own components. To make this board minority/few of the components which supplier uses are owned by customer. It means few of components which supplier uses to make this board is ordered/funded by customer and stored at supplier location itself. Once board is ready, supplier would directly to end customer, it will never come to customer facility. Supplier ships to end customer and bills the customer for the board. As of now the components which is owned by customer which is at supplier place is tracked by emails and excels. Supplier when he bills he reduces the price of those components and bills them. As board never comes back to customer stock we cannot use sub con process.

Now customer

1. Wants to track stock movement of these minor components which he owns but at supplier facility.

2.Wants to track these boards movement as of now all this happens in emails and excels.

Any ideas how to handle this?