Try creating a delivery order from the sale order.
On the sale order click on the delivery smart button, it would bring up all the associated deliveries, you have the option to create a new one.
If there is only 1 associated delivery for an SO, then you open that transfer duplicate it to modify the qty
If you created a back order a 2nd delivery will be automaticaly created and related to the same SO. If you did not, well I'm not sure, but if you intend to send the 5 units from the begining just make sure to create a back order. That I would say is the best practice. If you need to invoice whatever you have already sent you can select that option, invoice upon delivered instead of upon ordered. I would say the only case where you don't want to create a back order is when the customer or yourself decide to reduce the agreed quantity of units on SO, and the same applies to the vendor and yourself on PO.
That said I think what you descrived was a valid way to solve a case when you did not create the back order.
EDIT: I was able to fullfill the request by going into the sale order, going into deliveries from the SO, once looking at the deliver order I created a new one, adding the remaining quantity, and then reinvoiced the order, which invoiced only the remaining items. Would that work for you?