What is the ruling on buying and selling currencies online, where the buying and selling take place in the same session, and immediate possession occurs upon sale?

There is no harm in trading currencies online if the prohibitions are avoided and the Shariah controls are observed, such as the قبض (possession/seizure) of the two عوضين (counter-values/exchanged items). Possession can be sensory (حسيًا) or constructive (حكميًا). Constructive possession includes bank registration, such as depositing the amount directly into the other party's account or via a bank transfer. If possession occurs immediately upon sale or purchase by adding the amount directly to the account, then there is no objection to it.