Commission discussions need to happen before an offer is made. It's not unusual for a buyer's agent to request a specific amount on a transaction and request the buyer to make up any difference between what the seller offer and the agreement. In your case though, asking after the fact is a poor way of handling things. How you proceed is up to you, but if you didn't sign a buyer's agency agreement in advance, I can't imagine that you are obligated to pay anything.
If you want further clarification on what may be happening, ask to speak to the agent's managing broker.