However: Never, ever, ever sign a blank agreement or contract. Never.
In case my previous point is unclear, let me try again: NEVER SIGN A BLANK AGREEMENT OR CONTRACT. It doesn't matter who's presenting it. Don't do it.
Contracts aren't meant to be "gotcha" traps, they are meant to provide written evidence of an agreement between parties. You did not agree to the terms that the agent added to the contract, but you have to take immediate action to demonstrate your disagreement. Otherwise, a judge might think that you had agreed with the terms but were trying to back out later for some reason.
So - right now, tell the agent that they are not your agent and that you want a signed release from the company. Monday, call the manager of the office and tell them what this agent did, and ask when you can expect a signed release from the company - within an hour would be an acceptable response.
All the best,
Sorry to hear you are dealing with someone who is unethical and unprofessional!
By the way, what additions or changes did he make?
Keller Williams Realty Partners