I knew I reset the GCFI’s until I got a call from a client I done sellers inspection for. I told me that his receptacles in the master bathroom did not work. He only lived a very short drive from me so I went over. It turned out that I did not re set the GCFI properly. There was no harm done and he was grateful for me coming out but it does go to show it does happen.
Whist your client does sound like an A Ares hole I would pay him, get him out of your life and at least he then hopefully not bad mouth you. Remember if the sale did not go through he will be in contact with a lot of Realtors showing the house to there clients. Who knows what he will say to them