Automate invoice validation after PO increase

Summary:

How can we automatically validate held invoices when the PO is increased via a new PR

Content (required):

Scenario:

1. PO has 1 000 Eur amount Open.
2. Receive Invoice of 1 500 Eur which is matched to the PO.
3. When validating, the invoice will get Amount billed exceeds amount received and Billed amount exceeds received amount by tolerance amount holds.
4. Raise a new PR for 500 Eur and add it to the same PO.
5. Validate invoice again.
6. Invoice will have the same holds.
7. Manually allocate invoice to both PO lines (1000 + 500)
8. Validate the invoice again and holds are now resolved.

We have two problems with this process that I would appreciate some help with.