Is it permissible to buy currently circulated paper currencies online from distant countries, knowing that mutual possession (taqabud) is a condition when buying circulated currency, and is there any sharia exception to the issue of mutual possession in this case?

Monetary currencies are subject to the rulings of sarf (currency exchange), hence immediate possession (taqabud) is required when exchanging different currencies. If old currencies are still in circulation, it is not permissible to sell them for another currency without immediate possession. However, if their circulation has been abolished, they lose their monetary status and become a commodity that can be bought and sold for more than their face value without the condition of immediate possession. Buying circulating currencies online, or in different countries, or because they are rare, does not permit buying them without immediate possession.