PO open but WBS element closed

Hello,

We have a business scenario where a Purchase order is raised against a Project WBS.

When the project is to be closed there is a validation that stops you changing the status of the project until all the commitments are closed, ie that there are no open GR's to be processed.

However, it will allow WBS closing if the GR is done in full.

Later the invoice comes in and is processed against the PO, but there is a price difference. The system wants to post this price difference back to the Project WBS but it is now closed, so the invoice cannot be processed unless the project is opened again.

This seems is not efficient.

Can we set up so that the WBS can not be closed until the PO is fully complete including invoices? or can we have the account assignment changed to code to Cost Centre??

Has anyone else come across this situation with a workable solution??

thanks in advance.