Question on small accident

Hello, have a question about the following situation: Car drives back at another car. At first glance, it can only be seen that in the victim the license plate on the car is crushed. Harmful and injured exchange numbers and agree to handle the accident privately without the police. Now the victim registers and gives 100 € for the damage to want to have and to have it repaired with a good friend (car of the victim has residual value of 500 – 1000 € … so got into the years – damage is not fixed with security). Harmful, however, is assumed that only the license plate was damaged and not as the victim thinks, is a dent in the bumper. How should one proceed? The 100 Euro pay even though the demands are not justified? — Can one have the injured person, if one pays the 100 Euro at least something signed, that he does not think that he wants more? A Is this a letter that he renounces any further demands? What does that look like? Ask for the quickest answer =)