It's no use telling him, he will say, add some of this and that, and try a course of treatment. This is the case with you. Anyway, the moves are different, and the central idea is one thing: pay and keep digging. Why are you still staring?
I have invested a lot before, and I want to stop here. I think that all my efforts may have been in vain, so I am not willing to continue to fill in.
Feeling cheated, I want to sue for a refund, but I think it is also money, which is time-consuming and troublesome.
So ... not only do you refuse to pay the final payment, but you should also ask him for an explanation.