SO, order 10.

Invoice the SO, override the quantity shipped to 5, and the BO quantity on the invoice should auto-calculate to 5. 

Post, updating the SO with the BO quantity.

Your PDF shows the full shipment, so credit the 5 that didn't go out, then fix the BO quantity after posting.

Either your form is displaying incorrect data (meaning someone adjusted the form code to display the wrong information under "shipped"  and "backordered") or you're actually marking the entire order shipped when it's not.

Something definitely wrong with your sales order form.  No way you could have shipped the complete order and still have an open sales order.  Also do not enter a back order quantity when creating the sales order. Let the system do that when you ship what you have.

I did a quick check and Sage100User and BigLouie are correct.  There is something wrong with your printout. 

You can edit the BO quantity in a couple ways, but never exceeding the Ordered + Shipped (invoiced) quantity.

No, we aren't shipping the order complete and we can't print paperwork to show what is actually being shipped or delivered at that time. I'm not sure if you saw the PDF I uploaded, but the example shows "ordered-10  shipped-10  back order-5" when we want it to actually show "ordered-10  shipped-5  back order-5". I hope that makes sense and who knows, maybe this isn't possible without some other steps or function in Sage that we aren't using.

Those numbers are impossible, unless your form is designed wrong.

You print what is shipped from the Invoice, not the SO... so perhaps that is the issue.  If you need to set up something to print from the SO, you're looking at customizing the form.

So maybe our process is the problem? We create the sales order which has four copies: sales order, purchasing, shop and customer. When our drivers deliver to the customer, the customer signs the shop copy and keep their customer copy. After the shop copy is returned to our facility, it is matched with the sales order copy and at that point, the order is invoiced.

The SO is not usually a shipping document. 

Typically you invoice as things go out the door (where short shipments are known), and you print an invoice / packing list with what is included.

Ship the sales order when the product leaves the building with the truck, and print the paperwork from the shipped document.  Do not post the invoice.  When the truck and paperwork returns confirm what was delivered and then post the invoice.  If the truck did not return, then move that invoice to a different batch before posting the paperwork for what was returned by drivers.