Ruling on trading in currencies

Dealing in currencies is permissible so long as the exchange
takes place in the same sitting as the contract is made. It is permissible
to sell euros for dollars so long as the exchange takes place in the same
sitting as the contract is made. But when the deal is concerning the same
type of currency, such as selling one dollar for two dollars, that is not
permissible because it is a type of riba.  In that case they should be of
equal amounts and the exchange must take place in the same sitting as the
contract if the exchange is concerning one type of currency. The evidence
for that is the report narrated by ‘Ubaadah ibn al-Saamit (may Allaah be
pleased with him) who said: The Messenger of Allaah (peace and
blessings of Allaah be upon him) said: “Gold for gold, silver for silver,
wheat for wheat, barley for barley, dates for dates, salt for salt, like for
like, same for same, hand to hand. If the types are different then sell
however you like, so long as it is hand to hand.” Narrated by Muslim, 1587. 

It says in Majmoo’ Fataawa Ibn Baaz (19/171-174): 

Dealing in currency,
buying and selling, is permissible, but that is subject to the condition
that the exchange be hand-to-hand if the currencies are different. If a
person sells Libyan currency for American or Egyptian or whatever currency
hand to hand, there is nothing wrong with that, such as if he buys dollars
for Libyan currency hand to hand, exchanging it in one sitting, or he buys
Egyptian or English currency etc for the Libyan or whatever currency hand to
hand, there is nothing wrong with that. But if there is a delay, then it is
not permissible, and if the exchange is not done in the same sitting, it is
not permissible, because in that case it is regarded as a kind of riba-based
transaction. So the exchange must take place in the same sitting, hand to
hand, if the currencies are different. But if they are of the same kind, two
conditions must be met: they should be of equal amounts and the exchange
should take place in the same sitting, because the Prophet (peace and
blessings of Allaah be upon him) said: “Gold for gold, silver for silver…”

 The ruling on currency is as mentioned above; if they are
different then it is permissible for the amounts exchanged to be different,
so long as the exchange takes place in the same sitting. If they are of the
same kind, such as dollars for dollars, or dinars for dinars, then the
exchange must take place in the same sitting and they should be of the same
amount. And Allaah is the source of strength. End quote.