Is it permissible to sell dollars for local currency through an electronic bank transfer, given a potential time lag for the amount to arrive, and must the price be received upon the amount's arrival, or can the exchange rate at the time of transfer be used?

Selling dollars for local currency is a "currency exchange" (ṣarf), and its permissibility requires immediate possession (taqābuḍ) at the time of the transaction, whether this possession is actual or constructive, such as when the money enters the accounts of both transacting parties.

The basis for requiring immediate possession is the hadith narrated by 'Ubadah ibn al-Samit (may Allah be pleased with him): "Gold for gold, silver for silver... hand to hand. But if these types differ, then sell as you wish, provided it is hand to hand." Modern currencies are treated with the same rulings as gold and silver.

Accordingly, it is not permissible to take local currency while the deposit of dollars into the account is delayed. However, it is permissible to send the dollars to your brother as a trust (amānah). Then, once they enter his account, the currency exchange can be executed by taking the local currency at the exchange rate of the day of the exchange, thereby transferring ownership of the dollars to him. This is supported by what is stated in "Kashshāf al-Qinā‘" regarding the permissibility of leaving money as a trust and then performing the currency exchange on it.