Not to take his side, but I bet he thought that the damage to the panel was caused by your guys being careless during demo. He might not have known that this damage occured prior to you guys being there. Whenever we demo around electrical, we are careful to not cut any wires (leaving sparky short at a critical area). Sounds like he should have had the electricians come in and do some of their demo prior to yours. Explain to him the situation, and if any of the damage was caused by your guys, settle up with him accordingly.