Hi,
The reference document in delivery doc is the reference document for the delivery. If the delivery doc is created against sales order, the reference document is order and if its a replenishment delivery, delivery is created against PO and the same will be displayed as reference document.
In your case, the sales order is not an immediate predecessor for the delivery and hence it can not be displayed as a reference document through configuration as of my knowledge.
Regards,
AKPT