Is zakat obligatory on money saved in a piggy bank when the amount is unknown, and how is it calculated, and is it permissible to pay it on behalf of relatives?

Zakat must be paid on money saved in a piggy bank if it reaches the nisab, which is equivalent to 85 grams of gold or 595 grams of silver, after a full lunar year has passed on it. If you do not know when it reached the nisab, then investigate and act based on your best judgment. The amount of zakat to be paid is one-quarter of one-tenth (2.5%). As for paying zakat on behalf of relatives, it is permissible if they authorize you to do so. However, it is not permissible to pay it on their behalf without their knowledge, because intention is a condition for the validity of zakat.