You don't need to be a broker to call the CAR legal hotline. Just call them.
However, you should know that a check is "an order to pay". I am not an attorney, this is not legal advice, however because you wrote them a check, I would be surprised if you did not have some obligation to make the check good.
OTOH, you may legally pay a principle to a transaction part of your commission. However, the question gets back to "why would you"?
My two cents is to talk with your broker so they are in the loop, but this is your problem. Try to negotiate a lower amount, explain your situation. Often giving somebody something works far better than trying to reneg on your word.