Are listing brokers required to disclose Variable-Rate Compensation?

Yes, listing brokers must disclose variable-rate compensation to potential cooperating brokers as soon as possible and they must disclose the difference between the two rates if asked.  Listing brokers must disclose the information to cooperating brokers before the client makes an offer. Cooperating brokers must then disclose the information to their client.
What is Variable-Rate Compensation?
The REALTOR® Code of Ethics defines a variable-rate compensation arrangement as a listing in which one amount of compensation is payable if the listing broker’s firm is the procuring cause of sale or lease and a different amount of compensation is payable if the sale or lease results from the efforts of the seller, landlord, or a cooperating broker.
Code of Ethics Article 3 Standard of Practice 3.4  
REALTORS®, acting as listing brokers, have an affirmative obligation to disclose the existence of dual or variable rate commission arrangements (i.e., listings where one amount of commission is payable if the listing broker’s firm is the procuring cause of sale/lease and a different amount of commission is payable if the sale/lease results through the efforts of the seller/ landlord or a cooperating broker). The listing broker shall, as soon as practical, disclose the existence of such arrangements to potential cooperating brokers and shall, in response to inquiries from cooperating brokers, disclose the differential that would result in a cooperative transaction or in a sale/lease that results through the efforts of the seller/landlord. If the cooperating broker is a buyer/tenant representative, the buyer/tenant representative must disclose such information to their client before the client makes an offer to purchase or lease.