All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the bank rule on currency exchange at weekends is to offer the currency at a higher price than the rest of the week, then this is not Riba (interest). The requirement for the permissibility of this deal is the constructive possession (which refers to a state where a person does not have actual possession but has the legal rights to control an asset or property object), and one of its examples is a bank draft.
The resolution issued by the Council of the Islamic Fiqh Academy stated:
“Some of the forms of the constructive possession recognized both in the Sharee‘ah and what is customary are enumerated hereunder:
i. Crediting a sum of money in the bank account of a customer, in the following cases:
a) Where a sum of money has been credited to the account of the customer, whether directly or through a bank transfer.
b) Where a customer contracts a sale of “Sarf” by purchasing a currency for another currency standing in his own account.
c) Where the bank, on order of the customer, debits a sum of money from his own account and credits it to another account, in another currency, either in the same bank or in another bank, no matter whether it is credited in favor of the same customer or in favor of any other person. But it is necessary for the banks to keep in view the Islamic rulings governing the contract of “Sarf”. If such crediting takes some time to enabling the beneficiary to draw the amount so credited, this delay can be allowed, provided that it does not exceed the usual period normally allowed in such transaction. However, the beneficiary of such crediting cannot deal in the currency during the allowed period until the crediting takes its full effect by enabling the beneficiary to draw the amount.”
Allah Knows best.