Issue with creating packing list and Invoice from sales order placed to multiple vendors.

Issue with creating packing list and Invoice from sales order placed to multiple vendors.

Hi all,
I am facing this strange issue.
I have sale order against which I back ordered to different vendors, shipment from one vendor is received while from other is still pending.
Now I want to ship the item from first vendor but to my surprise the sales order shows ON HOLD and I cannot generate invoice nor package list etc.
This happens when I use back order option.
I wonder If anybody have solution to this issue.

Regards
Bilal