It is all about "procuring cause." Let me see if I understand your situation...you asked a Realtor to set up an appointment for you to see a house. Did you see it with the Realtor? Did you notice then that there was a for sale by owner sign on the lawn, but the house is on MLS?

If the owner's existence on MLS was what brought the house to your attention and this Realtor set up the appointment and showed you the house, the owner is obligated to pay a commission to the Realtor if you purchase this house or might be liable to a lawsuit.

It is unfortunate that being on MLS and also selling the house on your own tends to cloud up responsibility and confuse the buyers. I will never show a house that has a For Sale By Owner sign on it for just that reason.

You don't have to work with anyone you don't wish to work with, but I believe procuring cause may have been established and this Realtor is entitled to earn a fee from this transaction.