What is the ruling on collecting banknotes for memorabilia, and is Zakat obligatory on them, and if so, how is its nisab and how is it to be given out?

The default ruling is the permissibility of collecting currencies, and there is no harm in it. As for its zakat: If it is for trading, then zakat of trade goods is due on it, by evaluating it at the completion of the hawl (one lunar year) and giving out a quarter of a tenth (2.5%). However, if it is merely a hobby and for collection, then if its monetary value remains, its value is subject to zakat. If it becomes obsolete and is no longer traded, then no zakat is due on it. Currencies that are still in circulation are subject to zakat if they reach the nisab (minimum threshold) and a hawl passes over them. As for obsolete currencies, if they are gold or silver, they are subject to zakat as gold and silver. If they are made of other materials, no zakat is due on them unless they are for sale and their value reaches the nisab, in which case they are subject to zakat as trade goods.