Validating Freight Charges

In order to exclude freight from the discount taken on an invoice, we allocate it instead of matching it to the PO to create a freight line on the invoice.

Because of this, the end user cannot control how much freight is paid on an invoice.  Instead users would like the freight to be matched to the PO.  Has anyone found a way to match freight to a PO and still exclude freight from being included in the calculation for discounts?