Just an opinion not the law. If everything needs to be agreed in advance then you are advocating for more contracts and more attorney fees and more legal bullshit. Maybe where the system needs to go is to more of a fee based scenario. You want to look you need to pay an hourly rate. Every realtor elects to work in this manner just like an attorney or CPA and less like commissioned car salespeople, copier sales, insurance agents etc.
Unfortunately what this does is put people in the mode of trying to limit the amount of time the would spend with their realtor and possibly cost them the home they were actually looking for.
I don’t have all the details of your relationship with your realtor. Additionally I don’t have his side of the story either. So their is no way to know what is right or wrong. Legally, I believe you could probably tell him to pound sand. Morally I don’t necessarily think that is wright if he put in a lot of time.
He has no leverage in this situation so he has no right to think you owe him a full commission much less anything else. He is holding the knife by the blade not the handle.