Applly PPV to COGS of Drop Ship Sales Order

The COGS for a drop ship sales orders is the purchase price received.  But is the vendor invoice is received with a different amount  we would like the PPV to be applied to the COGS of the sales order.  We have setup COGS to use the customer location sales subaccount and set the PPV to apply to the Inventory Account,  When we process the AP Bill it creates the Inventory Adjustment but it cannot be released until the Sales Invoice is released   - no idea why this is required because the COGS was the PO cost and not the PO cost + PPV.