Dealing in currencies is permissible so long as the exchange takes place in the same sitting as the contract is made.
There’s even a site that lists all the forex brokers who list all the brokers who have an Islamic account which waves the swap or rollover fees at the expense of a wider spread.
Here’s Sheikh Hacene Chebbani talking about the subject:
Sheikh Hacene Chebbani was born in Algeria and has been living in Canada since 1997. Sheikh Hacene has completed a Master’s in Islamic finance (2012) from UK. In 1993 he graduated in Sharīah (BA) from the Islamic University of Madinah. While there he took the opportunity to study Aqīdah, Fiqh and Hadīth with some of the notable scholars of that blessed city.
In Canada Sheikh Hacene has worked in Islamic schools teaching the Arabic language and Islamic studies and has served as Sharīah consultant and head teacher for over 6 years.