The additional discount implemented from ‘ES15 ESDI Commercial Cash Discount’ specification and according to that,

“The additional discount concept introduced to handle “early payment discount”, meaning that the discount is given when the amount is paid cash (cash sales). Therefore, it is applied to a customer order or Sales Quotation header. The default value of the additional discount should be zero in both the Sales Quotation/customer Order header. Always an additional discount % should not exceed 100% in Quotation/Order. Also, it should not be allowed to enter negative discounts.”

The negative discounts in general in Customer Order line\Sales Quotation, this is a requirement from the Service Management. But never be a question that negative Additional discount is required for them.

This has been further explained by the product development team in the solution: 271874 and this could be a future enhancement request.