Is the money saved by the father in bank investment certificates considered unlawful, and is it permissible for the son to use it for his marriage, or take it as a gift or a loan?

The ruling on investment certificates depends on the bank with which one deals. If the bank is usurious, then the investment certificates are usurious. The aforementioned investment certificates are usurious because the profit is predetermined. Usury is a declaration of war against Allah. Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of usury, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged."

If the father was unaware of the prohibition of usury, there is no sin upon him for what has passed. However, after becoming aware, he must dispose of all usurious gains, and he is only entitled to his principal. He may spend these usurious gains on the poor, including a poor son if his maintenance is not obligatory upon his father. There is no objection for the father to give his son from his lawful money. The father must be convinced to get rid of illicit money.