Customer Discount via Credit Note Sales Order

We are processing customer discount through credit notes which might be issued after may be 10 days. Now the issue is that we are using the same item to give the discount. It is to show that if we purchase 100 items, we give 5 items as discount. So the customer need to pay for 95 qty.

We follow this method as we cannot provide the discount during the initial sales order.

Now, this method has a disadvantage. It is will reduce the qty by 100 ( during sales update) but it will increase the qty by 5 once we do the sales update for the credit note sales order.