Link Sales Order to Vendor Invoice

I'm trying to link vendor invoice information (specifically shipping information) to sales orders in a HANA Live system.  My client has a requirement to compare the freight billed to the customer to that billed by the vendor.  In addition, the client wants to have both open and cleared freight.  If I understand things correctly, the DMBTR field from the BSIK and BSAK tables should give me the freight charged by the vendor.  The problem is that I can't find any way to join this data to the sales order item data in order to compare the freight billed to the customer to the freight billed from the vendor.  Can someone please provide some information about how to link the information in the BSIK and BSAK tables to the sales order line item data in the VBAP table?

Any help would be greatly appreciated.

Thanks,

Rob