Is it permissible to give Zakat on behalf of another person from the giver's own money? And is it permissible to estimate different currencies in local currency at the time of giving Zakat to determine if the minimum threshold (nisab) has been reached, and then to give the Zakat in local currency?

A father is permitted to disburse his son's Zakat (definition) if the son deputizes him to do so. As for different currencies, they are combined to complete the nisab (definition), and Zakat is paid on their value if they reach the nisab. It is preferable for the Zakat to be in the currency that benefits the poor person. If the different currencies reach the nisab (equivalent to 595 grams of pure silver), then Zakat is obligatory at one-quarter of one-tenth (2.5%), and there is no objection to evaluating and paying the Zakat in another currency if that is more beneficial to the poor person.