What is the ruling on buying and selling dollars remotely through bank transfers to benefit from price differences?

Selling currencies is called currency exchange (sarf), and its permissibility is conditioned upon immediate possession (taqabud) in the contract session, whether it is actual or constructive. Among the forms of constructive possession (qabd hukmi) that are recognized legally and customarily is the banking entry of a sum of money into the customer's account. This includes: if the bank, by order of the customer, transfers an amount from his account to another account in a different currency, for the benefit of the customer or another beneficiary. A delay in the banking entry for customary periods is overlooked, provided that the beneficiary does not dispose of the currency during this period until he is able to take actual possession. If the two amounts are transferred by the exchangers simultaneously or successively without delay, then constructive possession has occurred, and the transaction is valid.