Accidental

Hello I have a query regarding accidental case. Last year 4 months ago my uncle was driving back from his job to home. The road is kind of a highway. Suddenly a man arrived and got hit by his car. He got injured seriously however my uncle stopped and reached the nearest government hospital as soon as possible. The injured man was totally drunk and after accident he was unconscious. My uncle waited until his family reached. And after that he spend for all his treatment , medicines , surgery , medical bills. After discharge from the hospital he also helped him to his home. He bought the medicines and gave 8000 separately if by change they needed it. But within 2 weeks they asked for more money. He gave them 5000 later. However both sides have already signed mutual agreement with 3 witnesses from both sides. They always call back for more money and says the agreement was not signed with their agreement. Even after visiting him they lie that he didn't visited him and didn't gave 8000 ever. Now they try to blackmail him by saying that they can do anything if they want. So he has decided to meet them together in the same police station and settle it down because the money was given to them at their home without the presence of police because of which they always call back the police station claiming he never visited or given any money. Please if anyone could suggest what would be the better procedure for settle it in good note. How much money he should give to him to put this to end. So that after full and final compensation they could not blackmail him back. Thank you