Is it permissible to demand the return of a gift given to a relative, which was a donation whose true value was unknown at the time, but then it became apparent that it is of very high value?

It is not permissible to retract a gift, according to the Prophet's (PBUH) saying: "The one who retracts his gift is like a dog that returns to its vomit." An exception to this is a father retracting a gift to his child.