Read your listing agreement carefully and then stand your ground.  If the agent is up to some funny business you need to report him by any means possible.  If they are a licensed broker in your state report them to the state licensing bureau.  And the state Attorney General's office.   Obtain an affidavit from the buyer that there was no one else involved in the sale.  And print out the agreement and anything from the website that may be helpful.  Good luck.