I say, and with God's help: This is a matter of selling rights. You have established a right to repair the car, so it is permissible for you to take an amount of money in compensation from a person who is trying to collect it from the insurance company. If this person manipulates the situation to take an excess that he does not deserve, the sin is upon him, not upon you. However, this is on the condition that such actions you are taking are lawful, and God knows best.