The letter you were given by the other party acknowledging payment and that he wouldn't open a case will provide you with evidence that the case was settled and will not be reopened. When a case is settled, the parties sign a release of liability which prevents a future lawsuit from being filed on the same claim.The only potential problem here is if the payment he received from you was for property damage to the other driver's vehicle and his new claim is for personal injury, those would be handled separately.  If he does have a personal injury claim, he can file it with your insurance company