Real Estate Law USA.com

I am taking a test tomorrow my final for my real estate class and can not find this answer. When signing an exclusive right to sell contract, a provision can NOT be added. There is a law against it. I’m not sure if it is a federal law or a state law. I am in North Carolina. Can anyone help me with this? What is the provision that can not be added and what is the law against it? I have searched my book and can’t seem to figure this out.