Currently, under both the Vendor Page and Products Page, there is a 'Tools' Button on the upper right hand side...

When you click on the Tools Button a drop down box appears where you have the ability to select ' Generate a PO for  the products below the 'Reorder Level'. On the following page, it asks you to select a Vendor and a Pick-List is available that lets you select from a list of Vendors who have products that are below the 'Reorder Level'. When you select a Vendor all of their products that apply appear in a list below that lets select what you want to generate a Purchase Order for.

Here's where the idea comes in...

What if you had this tool available on the 'Sales Order Page' where when you select a checkbox next to your Sales Order, then click the 'Tool'  where it allows you to select ' Generate a PO for  the products on this Sales Order. On the following page, it asks you to select a Vendor and a Pick-List is available that lets you select from a list of Vendors who have products that are on the existing sales order. When you select a Vendor all of their products that apply appear in a list below that lets select what you want to generate a Purchase Order for. This way, if you have products in stock, and you see from the list that you have enough to cover the order, you'll know that you can partial ship from stock and generate a Purchase Order to the Vendor for the Remainder of the products. 

See Image Below for Mock-Up of Idea...