I don't know how to politely say this, but: It's very important to READ and UNDERSTAND anything you sign. I see a lot of questions here on Trulia. In some cases, the people admit they didn't read the documents. Or they didn't understand them. Or--as in your case--there isn't a mention of whether there were documents.
Sorry, but if someone signs an agreement promising to do something, then they've got to do it.
Again, I don't know if that's your situation. Now, if you didn't sign anything with the Realtor, then no, you don't owe him anything.
So: Read anything you may have signed and proceed accordingly. And if you didn't sign anything, speak to the agent's broker.
Hope that helps.