No. According to MLS rules, listing brokers must specify the compensation being offered to cooperating MLS participants. These offers are unconditional unilateral offers. The unilateral offer becomes an enforceable agreement between the brokers when a seller and a buyer execute a purchase contract and the cooperating broker is the procuring cause of that sale. Although the brokers can mutually agree to modify the cooperating compensation, neither broker can unilaterally change the compensation as stated in the MLS.
In addition, Standard of Practice 16-16 under Article 16 of the REALTOR® Code of Ethics states that a REALTOR® acting as a buyer's representative may not use the terms of an offer to attempt to modify the listing broker’s offer of compensation or make the submission of an offer contingent on the listing broker agreeing to modify the compensation.