Quantity differences between goods receipt and invoice 
receipt for a purchase order result in a balance on the 
GR/IR clearing account.

If the quantity invoiced is larger than the quantity 
received, the system then expects further goods receipts 
for this purchase order to clear the balance.
If the quantity received is larger than the quantity 
invoiced, the system then expects further invoices for this 
purchase order to clear the balance.
If no more goods or invoices are to be received, you must 
clear the balance manually.

This can be done in different ways:

You can return the extra goods to the vendor
You can cancel the invoice.
You can clear the GR/IR clearing account manually.