What is the ruling on someone who officially gifted a house to another person, granting them full disposal over it (selling or renting), but then years later the donor wanted it back and the other party refused? And is the refusing party sinful for not returning the house?

If the donee takes possession of the gift and seizes it, then it is complete, and he is not obliged to return it to the donor. He does not sin if he refuses, rather it is not permissible for the donor to retract it unless he is the father of the donee; due to the saying of the Prophet, peace and blessings be upon him: "The one who retracts his gift is like a dog that vomits and then returns to its vomit," and due to his saying, peace and blessings be upon him: "It is not permissible for a man to give a gift and then retract it, except for a father in what he gives to his child."