Are the profits obtained by the father from platinum investment certificates with a 20% return permissible (halal)? And if they are impermissible (haram) and the father refuses [to acknowledge this], is it permissible for the son to give the amount of the interest on his behalf to the poor with the intention of purification?

Savings or investment certificates with a return attributed to the capital are usurious and forbidden, because profit should not be a percentage of the capital, but rather a percentage of the profits. If guaranteeing the capital is combined with a percentage of it, this is a forbidden usurious loan. Jurists have unanimously agreed on the prohibition of specifying investment profit as a lump sum or as a percentage of the capital. The father must be advised by explaining the impermissibility and danger of usury, and if he does not respond, then giving away the interest amount does not remove the sin of usury. If the father insists on the forbidden transaction, then the money can be given away with the intention of charity and its reward dedicated to him.