Hi julieanne-juille,
You can run a Sales by Customer Detail Report to show discounts you have applied to a particular invoice. In this report, each product or service pertaining to an invoice will be shown in separate lines, and the discount will also be listed on a separate line.
If you don't want to group the sales by customer, you can select Group by in the top left Rows/columns section and then select None, and Run report.
Hope this helps!
Bonny