release letter sample?

We had a minor car accident, the other party has agreed that we pay them a certain amount (1000+) of money then we are done. We don't want to turn this in to the insurance. My agency mentioned I can have a release letter sent him with the check saying something like "upon cashing this check, xxx is released from any further obligations and claims from the car accident happened at ... when ...".

Have anyone have this kind of experience? Would someone kindly provide a sample letter I can use?

Thanks very much!