Is currency speculation through the bank, by converting local currency to dollars and then back again after a period to profit from the difference in buying and selling prices, permissible and does it fulfill the requirement of "qabḍ" (possession)? And what is the correct method for trading currencies if this transaction is invalid?

It is permissible to trade currencies on the condition of taqabud (mutual possession/exchange) at the session, based on the Prophet's (peace and blessings be upon him) saying: «(Exchange) gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt, like for like, equal for equal, hand to hand. If these types differ, then sell as you wish, as long as it is hand to hand.» Taqabud can be actual or constructive, such as depositing in an account. It is also stipulated that one trades only with their own money, without using margin.