Handling overshipped quantities

Hello Again All

I am looking for some of your best practices when your teams receive too much of something and processing payments/credits.  Example:  We order 10 EA of item XYZ.  The PO says 10 EA of item XYZ.  The vendor sends 15 EA of item XYZ.

There is a constant struggle between the Storeroom and our AP team.  Referencing the above situation, our Storeroom believes the best practice is to receive the 10 EA of item XYZ and send the remaining 5 EA back to the vendor.  AP says this is wrong because the invoice they get from the vendor has the incorrect amount on it.  The invoice will list 15 EA.  Then, there is confusion about the 5 EA.  

Our AP group says the best practice is to receive 15 EA of XYZ, create a Lawson Return number (PO31, if I have that number right) for the 5 EA and send the 5 EA back to the vendor.  The invoice for 15 EA is paid, and then a credit for the 5 EA needs to be followed and processed.  The Storeroom says this is incorrect because they should not be accepting more quantity than what was ordered.  Another reason is because the Storeroom does not want to accept responsibility for the overshipped quantity.  Bringing the whole amount into Lawson shows accepting responsibility.  Finally, they have had instances where they shipped the overshipped quantity to the vendor.  The item was damaged going back to the vendor, and had to pay for an item that was never ordered to begin with... it was an overshipment.

So...  in a attempt to see what is the best practice....    what do you do at your facility?  Do you ship the extra quantity back immediatly?  Or, do you process all that was received (including the overshipped amount) and hope AP catches the credit and applies it to the correct accounts?

Many thanks in advance

Den