I think you can get out of this pretty easily. One of the agents here can probably help you push back. Once the agent sees you have enlisted the troops to fight back, he’ll run.
If you really get stuck on this, I think you might consider asking for a seller-signed document that shows proof that the seller truly “won’t sign the cancellation of contract document unless (you) sign an exclusive representation agreement with (your agent).” i.e. make the seller sign something that says they will sign the cancellation contract if the buyer signs an exclusive contract with the agent. Make sure they agree to sign it within 7 days.
You can claim that, without something written, you are signing an exclusive contract with no proof that there is benefit for you. So you want the seller to sign a declaration that he will, indeed, let you out of the deal if you sign the exclusive contract. I think this is a way to call the agent’s bluff.
You write the statement and give it to your agent to have the seller sign.
You know you’ll never see that doc come back, and it forces the issue as to whether or not your agent is being level with you.
If you do, indeed, get that document back from the seller, make sure it says “exclusive contract” but does not have a length of term requirement on it. Sign the exclusive for 8 days, then bail.
Enlist legal help on this, though. I’m not a lawyer and who knows what kind of traps there are in the law on this.