Salam Aleikom, Islam has put two rules when exchanging currency, which one of them is that it should be hand to hand transaction. These days people are using internet and mobile applications, this will raise two questions:
1- Are such type of transactions are considered as hand to hand? Where the money is being exchanged in your account and not yet withdrawn and when you do, you'll receive your money based on updated currency rates. Is this Hand to Hand?!
2- Isn't participating in such applications and r=earning or losing in matter of seconds based on flow of market a type of gambling?! Since one doesn't know how prices flows.