Am I obligated to pay zakat if the amount designated for marriage is depleted before a Hijri year has passed? And is the money I gave with the intention of zakat, without its conditions being met, considered money for purifying the usurious gains I unknowingly acquired?

The legal nisab for gold is 85 grams of pure gold, and for silver, it is 595 grams. If wealth reaches this amount, zakat becomes obligatory. As for money given as charity before reaching the nisab, the one who gives it will be rewarded for it. If the wealth falls below the nisab before a full year has passed, then no zakat is due on it. The year is calculated from the time the wealth reaches the nisab. As for money deposited in usurious institutions, the usurious interest must be disposed of by spending it on Muslim welfare projects or giving it as charity to the poor and needy. It is not counted as zakat. The grandfather should be advised that dealing with usurious institutions is forbidden.