Is it permissible for a father to give his son money earned from usury with the intention of charity, or from his zakat, to purchase a medical device the son needs, knowing that the son's income is moderate and barely sufficient for him?

The obligatory maintenance for a child ceases to be incumbent upon his father once the child reaches adulthood and is capable of earning, according to the majority of jurists. If the child's maintenance is not obligatory upon his father, then it is permissible for the father to give his Zakat wealth to his indigent son, as anyone who does not find what suffices him is among the recipients of Zakat. The same applies to ill-gotten wealth, such as usurious interest; it is permissible to disburse it to the poor and needy to the extent of their sufficiency.