Short answer is “Not Possible” using out of box functionality.

please see the below post and read through my responses:

Also keep in mind Acunatica rules doesn’t support both side dynamic field comparison (I see it is a limitation but they have developed as is) rather it compares a dynamic field from THE CURRENT ENTITY to a parameter (dollar value, …) not formulas or other fields

Hi @charltonh the approvals are based on fields on the screens. What if you add a user defined field to the Bills and Adjustments screen and use a business event that would update it using an import scenario and you could define the logic using a GI for your requirement where AP Trans unit cost or ext cost is > than PO issued amount