A Cash Discount can be added either to the Final/Invoice Project View or directly to the Invoice.
To add cash discount to the Project View enter the amount in the Rate Field. The discount will only be applied if the entire Project is closed and invoiced.
Create an Invoice and add an Un-Referenced Object. Select Type > Price Operator and enter the Discount as minus amount in the Sell field. Change the icon and name. The discount appears as last line in the invoice.