Much of it has to do with the contract. The standard Massachusetts Exclusive Right to Sell has a clause stating that the original listing agent would receive the commission if the offer came in from efforts of that agent and if you have not signed with another agent.
Jeannie, This is definitely a gray area...Technically you no longer have a contract so you should be able to have this offer go through a your new agent but, I would suggest consulting a real estate attorney.
Take a look at the contract you signed with the first agent and see how it handles this situation. If the buyer saw the property while it was under contract with the former agent then there is a period of time during wich you need to honor that contract.