What is the ruling on buying a house from a maternal uncle who claims he inherited it from the grandmother through a gift, but there are doubts that he took it from her by fraud because of an illness that affected her mind, and is the money acquired from this sale considered unlawful?

A Muslim must think well of his Muslim brother. If the maternal uncle states that the house was a gift from his mother, then the default is to believe him.

However, if the mother was of unsound mind at the time of the gift, then the gift is invalid, because the actions of one who is of unsound mind are not enforceable. But if she was in full possession of her faculties and health, and he has a valid contract, then he should be believed unless there is strong evidence to the contrary.

If the mother had other children and did not treat them equally in the gift – meaning she gifted to her son but did not gift to his siblings what she gifted to him – then scholars have differed regarding the ruling on this gift: Some say that it is enforceable and the remaining heirs have no right to revoke it. Others say that it must be returned to be divided fairly among the heirs. This is the choice of Shaykh al-Islam Ibn Taymiyyah and Shaykh Ibn Uthaymeen, who hold that the one who was favored must return the excess so that it can be divided with the rest of the inheritance.