Trading in currencies is permissible, such as buying the dollar at a price, then selling it at a higher price, and so on.
Likewise, it is permissible to sell it for the same at a higher price with the sale being present, such as selling a note of one hundred dollars, for ten bills of the same ten, and five more than it, so the value is 105, provided that the sale is not deferred for a term.
As for electronic currencies, His Eminence Sayed Sistani does not allow dealing with them at the present time, as an obligatory precaution.