AR Invoice report shows the discount 

Products flagged as Discount Products can be added to AR Invoices to discount one product, a product subtotal, the entire invoice, or to any combination thereof.

AR Invoice Discount Products do not change the price and have no effect on product cost, therefore they have no effect on product margins.

Taxes cannot be discounted, and individual products can be flagged as Exclude From Invoice Discounts.

I want to read the white paper about how to setup and use AR Invoice Discount Products (with much more detail, lots of screenshots, and plenty of examples).