Currently, Sales By Customer Detail Report is showing a net amount of discount. For example,
Report that filters out the discount should show:
There is an enhancement filed for it under 141955 - Reports>Sales>Sales by Customer Detail– Sales by Customer reports should show Sales amounts excluding payment terms discount.
You can try below workaround for this:
1. Click Customize button or Return to Customization > in the Report Builder Page > Edit Columns > Expand the Sales Transactions folder > Add Amount (Gross).
2. In the Filters page> Expand Sales Transaction folder > Expand Item sub folder > Select Item Type is not equal to Discount.
3. Rename this Report and click Save.