It sounds horrible!!!

Put everything in writing and get a signature and verify ID just in case.
Somewhere i saw that you should pay in thirds - a third to start, a third @ halfway point and final third when job is finished and acceptable to you.
If not acceptable you can renegotiate the final payment or he can do it to your approval..
from court tv shows

hubby, kids & I do all our ow remodeling & repairs
Well the kids helped before they got jobs anyway.
But we get a real sense of accomplishment doing a project ourselves.