Delivery-related billing for TAS : take VPRS value from GR or Invoice

Hello,

One my customers currently invoices stock items and Third-Party Items separately. He now wishes to invoice both items together.

In Third-Party standard flow no delivery is needed. In order to invoice both items together I have now activated the delivery for the Third-Party item flow (item category TAS).

I can now invoice both items together based on the common delivery.

I have now another problem regarding the TAS item invoiced...

- In standard flow (third-party processing = order-related billing), the value for VPRS is taken from the MM flow (either from Goods Receipt or from vendor's invoice depending on the customizing) automatically. This is great since the vendor might enter a different cost than the one from the sales order and it is that cost that we need to find in the billing doc and in CO-PA.

- When I do delivery-related billing for my TAS item the VPRS is not updated from MM but is taken from Sales order (thus from material cost) --> incorrect value for my customers's need

I need to do delivery-related billing for item TAS with VPRS in billing doc (SD invoice) = value of MIGO or MIRO

Can anyone indicate me how to do this (user-exit? routine? ....)

Thank you

Regards,

Cynthia