The ruling on buying and selling Currencies
Question:
Is it permissible for the Muslim to buy dollars or other currencies at a cheap price and then, after the price goes up sell them?
Answer:
There is no harm in that if he buys the dollars or any other currency and keeps it with him. Then he sells them after that when the price goes up. There is no harm in that. However, he must buy them hand to hand (by paying up front) and not on credit to be paid later. He buys the dollars with Saudi riyals or Iraqi dinars (for example) hand to hand. Currency must be paid for up front just like buying gold with silver, hand to hand. And Allah is the One Who is sought for help.
Source:
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Darussalam Vol:4 page no.415