How to deal with false allegations of fraud

My father bought a land from a person who was perfectly normal at the time of registration of land. He payed them using a cheque and not cash which is to be noted to state that he was clean. Now, the brother of the person from whom my dad bought the land claims that the person is blind, deaf and dumb, and also treatens saying that my dad has engaged in fraudulance by taking advantage of the person's physical inabilities. Which thoroughly did not take place. Is there any way he could come out of this problem?