Hi there
First you create an option with an exercise date e.g. August 30th. You settle the transaction and post the premium. When you come to the due date you have to exercise or expire the option. If you exercise the option automatically an FX transaction is generated. You settle the fx transaction and post it. This is the process.
If the system does not do it please check in customizing for derivatives (76a in standard SAP) 60A product type and transaction type 101 in the chapter "underlying".
Regards
Juerg