In our company we are using "Three-way matching" when doing the invoice validation.

Consequently, we are only posting Vendor Invoices where:
- Quantities have been posted as received
- Prices/Amounts of the Vendor Invoice are in line with the Purchase Order

We have the following scenario:
- We are ordering an item with a particular Qty from our Vendor (e.g. 10 Units)
- Due to several reasons, the ordered quantity is not received at once (because of space restrictions on the truck or supply difficulties on the Suppliers side – e.g. first delivery contains 2 Units) >>> We are posting the product receipt of the actually received Qty.
- The invoice is now received for the first delivery (e.g. 2 Units). In our example the price for that is e.g. 300 €. We are posting that invoice
- Now the Supplier can deliver the next Units (e.g. 3 Units) – thus we are posting a new Product receipt (with Qty. 3)
- We are now receiving the Invoice. As the price for the ordered item is variable the invoiced price is now 305 €/Unit. However unfortunately we are not able anymore to change the price of the Purchase Order as it is already partially unvoiced - the field is not editable anymore.

From my point of view this is a bug / lack of functionality as there is no way of solving this issue in a standardized process.

It should be possible to update the Unit Price of a PO Line even if it is partially invoiced