From the way he says "maybe I made it too steep", I aasume that the design is his instead of yours. Therefore, he is the one who made the mistake, not you. You should not have to pay for his mistakes. This may be why it is best to deal with strangers and get iron clad contracts. As uncomfortable as it may be, I would be just as "unfriendly and businesslike" as he is being.
By the way, this recently happened to me on fencing. I have learned my lesson and my wife is reminding me of it.
