I hired a roofer to fix my leak on the roof(three story building). He seal it from attic and will charge $1000. I am not familiar the market and agree to pay him with $750 with cash. He just worked around one hour. After that, I realized that he charged me too much and asked $300 back. He said he could only refund me $250 without warranty. I think this is not fair for customers. How can I do that?

You should have had a written contract with a set price. If you agreed to pay him a set amount then that is what you owed him. Sounds like you re down to $500 in stead of $1000. I'd stop right there and let it go.

Whatever he did it was for half price, and no matter how long it took if it worked you may have gotten a deal. Regardless since you had no contract, nor experience yourself, next time get 3 FREE estimates from reputable contractors BEFORE you agree on a price, or have work done.

I would have taken you to court for the rest of my money.

Agreed, you get a firm signed contract prior to him starting about hourly charges, service charge and so on.