I am working with a client integrating with a TMS.  They are integrating sales order, purchase orders, intersite, intercompany, customer returns and supplier returns.  In each case, the information from the originating document is sent over to the TMS for processing.  In this client's particular case, there is not much variation between what is on the sales order and what makes it to the delivery.

Since the information between Sales order and delivery can be very dynamic, it seems to me that the earlier you can allocate inventory to the sales order the better the chance for the delivery to match the sales order allocated quantities.  The other option is to reach send the information to the TMS based solely on the delivery.  The turn around time seems to be pretty quick (hours).

Bottom line, as you are discovering, a lot if business rules need to be considered and applied.

Hope this helps.

P.S. - A colleague also reminded me that another option depending on the process again, information from the pick list might be the way to go.  That is, create the sales order, create a pick ticket and send the information from the pick ticket to the TMS.