Are the bank interest earnings resulting from depositing another person's money into the father's account — which the owner of the money left as a gift for the son — considered permissible (halal) or forbidden (haram), knowing that the father does not want them?

Usurious gains are forbidden, and whoever acquires them must dispose of them by giving them as charity to the poor or for the general welfare of Muslims. Ibn al-Qayyim held that money acquired from forbidden means with the consent of the payer and the receipt of its forbidden عوض (substitute, consideration), such as someone who trades in alcohol or fornication, does not have to be returned to the payer. Rather, the recipient should give it as charity. This is also the view of Ibn Jibreen regarding bank interest earnings. They must be spent on charitable causes for the poor, the needy, the mujahideen, and similar beneficiaries. As for gifting them to those who are not poor, this does not constitute disposal.