Prevent AP Bill from Posting to Purchase Order when AP Bill Total is greater than PO Unbilled Amount

Does anybody know how I can have Acumatica prevent one from Posting an AP Bill against a Purchase Order if the Amount of the AP Bill is greater than the Unbilled Amount of the PO? There is a setting in the Purchase Order Preferences screen where you can choose between No Validation or Validate with Warning but the Validate with Warning still allows you to do it. Our client wants this to be prohibited. I thought of using an Approval Flow to require approval if the AP Bill Total Amount is greater than the PO's Unbilled Amount, but I don't see how to do that. Is there another way?