You can only create a journal transaction in base currency. If you want to record a bank service charge in foreign currency, you may consider to use a Payment transaction instead of a journal transaction.
In the Payment transaction, select the foreign currency bank account in the “Bank” field, and select the relevant expense account (such as Bank Fee) in the By Account tab. The amount entered will be a foreign amount (such as USD).
Assuming the Bank Fee is USD 30.00 (based on exchange rate of 0.8USD to 1SGD), the double entry in this case will be:
Debit Bank Fee 30.00
Credit USD Bank 30.00
Debit Bank Fee 7.50
Credit USD Bank (Currency delta) 7.50