Of course the issue here is the thorny one of procuring cause and that may change from state to state. You need to consult an experienced real estate attorney.
As a practical matter there are a number of solutions, all dependent upon cooperation and approval of your client, whose best interest must be paramount. Here are a couple:
(1) Inform listing agent that before proceeding you need him to acknowledge that regardless of who is procuring cause, he waives any claim to commission. He may still claim that he agreed under duress, but so be it....the fight is on! You may want to mention a few alternative properties you will be showing your client.
(2) Any agreement signed must be signed by listing broker and listing agent and you and your broker.
When you say you are still the official buyer's representative I assume you mean you have a current binding exclusive buyer's agency agreement. If so, you may have a right of action against your client (I would NEVER advise this) for the commission or other compensation, depending on its terms.
Whatever you do, do it in consultation with an attorney.
Sep 7, 2010