AP Invoice Approval for Foreign Currency Amount

Summary:

Client Base Currency is USD and we have AP invoices that are in different currency. How should I create a rule for this in the approval worksheet?

Currently I tried using the following:
- for Forex Invoices Base Amount between 0.00 and 20000.00, Invoice Amount = 0.00
- for USD Invoices Base Amount = Null, Invoice Amount = 0.00 and 20000.00
It is successful for forex invoices but failing for USD invoices.

Is there a parameter I can use that always returns the Base Amount regardless of the currency? From my understanding if invoice is in USD, the Base Amount field would return as Null?

Thanks
Lyndon