Deb's suggestion that your parents offer to pay out of pocket expenses that the agent incurred would be a nice gesture.
Good luck, to you,
Unwavering Commitment to Service
There is such a thing as "excluded prospects," but I think that "stiffing" the Realtor(r) now that it's on the market is a bit, uh, stiff?
Keller Williams Realty
If they need professional services of some sort (and they probably will), it's possible the Realtor would do it for a reduced fee. Or you could use a real estate lawyer.
It would be a nice gesure, if the sellers offered to pay for any upfront expenses (postcards, brochures, virtual tours, etc)............but in no way would I expect a commission under those circumstances.
Have your parents speak to their agent.