Is it permissible to deal with a currency trader who converts the local currency into the currency of his own country for a small profit, and is it considered usury if the profit increases upon payment in installments for the same amount?

Exchanging currencies is called sarf (money exchange), and it involves two types of riba (usury or interest): riba al-fadl (usury of surplus) when exchanging the same currency, and riba an-nasi'ah (usury of delay) when there is a delay in the قبض (taking possession) of one of the two عوضين (counter-values). Paper currencies have the same rulings as gold and silver regarding riba and other matters. When exchanging one currency for another, taqabud (mutual possession) in the contract session is a condition. It is not permissible to buy currencies in installments because it leads to riba an-nasi'ah. Rather, taqabud must occur either actually or constructively, by the transfer of both currencies between the two accounts. Giving part of the amount and delaying the rest in sarf is not permissible because it leads to riba an-nasi'ah.