ISP Suppliers are unable to add negative value lines

ISP suppliers do not have the ability to add a misc. line and enter a negative value (discounts, allowances, savings)

Suppliers often offer additional savings, outside of discounts, which are not included in the PO price. If a supplier is submitting their invoice through ISP and they have a line amount that is for an allowance or savings, they cannot enter a negative value. Is this by design or is there a configuration setting that can be enabled that would allow suppliers to add a line and enter a negative amount?