Hi Kenneth: I'm going to chime in to agree with several of my professional colleagues. The answer to your question is in the contract that you signed, or in the offer that you submitted to the sellers. When you signed those documents, you entered in to a contract. We use that word as if it's something complicated and legal, but a contract is simply an exchange of promises. You say that you're going to do A, B and C, and the other party says that they promise to do D, E and F. Typically, a contract includes specific instructions as to what will happen if you or the other party don't keep your promises. We call those the "default provisions".
If one or more of the contracts you signed said "I will pay the commission plus $500 in the event that I cancel this deal", then the Realtor has the right to hold you to that promise. If the contract is silent about what would happen if you broke a promise, or the contract says something other than "$500 plus commission", then that's not a legitimate penalty from breaking the contract. No one can tell you what the agreed upon penalty was unless they see the language of the contract.
Read both the offer and any side agreement you had with your Realtor carefully, and look for the explicit default provisions. They'll tell you what the consequence was of breaking the agreement. If you feel that she is asking for something that was not covered by these agreements, by all means, you should ask her to defend her demands.
Hope that helps! Best of luck,
Villa G Realty, Inc.