In the System module>>Person register>>Sales Tab you can link a Person to a Minimum Price List. The system will not allow a price after discount to be less than the item price in this list.

I second way to do this, which will requires a small customisation, is to add a Maximum Discount field onto the Person Register. A check must then be added to Orders, Invoices etc, which will check that the discount entered by a person does not exceed this Maximum Discount Set on the person register.

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