Tax Purchase Orders by Legal Entity

 We are configuring e-Business Tax to tax transactions for a single legal entity.  We have configured Sales Tax to allow user to manually tax lines in Receivables and the legal entity designation works fine since Legal Entity is selected on invoice in Receivables.  However, Purchase Orders will also need to be taxed for the same legal entity.  The problem we are having is that all Purchase Orders for the Operating Unit are being taxed.  The Operating Unit has 3 Legal Entities and we only want to generate tax for one of the legal entities in the operating unit.  Is there