You can refuse to pay any amount more than you feel is fair (or that you are comfortable paying): unless you are sued and lose, any payment by you is voluntary.He can take whatever action he deems is appropriate, whether that is suing you himself or filing a claim with his insurer (if he has relevant insurance; i.e. collision coverage), in which case the insurer *may* later sue you to recover any amounts it paid out to him. If you are sued, the person or company suing you has to prove your fault to recover money; you in turn can try to refute their case by providing evidence or testimony showing the other driver at fault, too. You can also, if sued, countersue the other driver for any damage or losses you incurred. The court will decide who was at fault, and how much, and apportion costs or damages appropriately.