Discount as separate line on sales order line

Summary:

We have below discount requirements from pricing point of view.

1) Sales order with 5 items, and total amount is $100

2) We add another item #6 named as "Discount" and the value is -$10

3) The sales order total amount is $90 (100-10)

4) We issue AR invoice with item 1 to 5 with total amount $100 to customer

5) We issue a credit note with $10 to settle the item 6 and close the sales order 

Generally we add discount at each line using pricing algorithm and discount list. However customer do not want to add the discount at each line but rather put it as separate line on sales order with negative amount and not to put that in AR Invoice