Using Shipment On Behalf Of

Shipment on behalf of is an order that issues stock to a customer from an inventory business unit on behalf of an order management business unit when each BU posts to a different general ledger BU.  We are trying to adopt this business process to replace direct ship from vendor, where our various BU's are set up as intercompany vendors and customers.  We have followed the pbooks instruction, Designing Shipment On Behalf Of.  The issue we are having is in how to solve the interunit transaction.  The inventory transaction generates entries for AR / AP under the respective GL's. There