The broker has to make a claim for commission, the agent cannot.
Where I practice in Las Vegas, the Buyer Brokerage Agreement is between the Buyer and the Brokerage, I do not believe the other agent is entitled to any of your commission. Consult with your Broker, I would venture to say their claim has no merit. This may vary from state to state, check the verbiage of both agreements if you can. In the past when we have had disputes over Buyer Agency agreements, we had to take up the dispute with the Buyer, not any other company they were doing business with. The other company may have no idea the client signed an agreement with another company.
Century 21 Infinity