@raJ, I notice you have not voted for the idea that would solve your problem: Delivery notes copy to sales invoice.
I also suggest that your problem could be solved by a change in workflow. Instead of beginning with a sales invoice, why not start with a sales order? From the sales order, you can make your delivery of goods. Once you know the exact quantity delivered, you can edit the sales order. Then, copy it to both a sales invoice and a delivery note.
You are always going to have the requirement of adjusting the quantity delivered somewhere in the system before sending the final sales invoice. The sales order gives you a place to do it that (1) does not have any financial impact and (2) allows copying to both delivery notes and sales invoices.