Just wanted to confirm that the advice given above is correct. Bought an Octopus card with HKD 150 of value plus HKD 50 of deposit. Did a <24-hour return (but not same-day return) with MTR connections on both ends. Received back the expected 100 HKD cash upon return to the airport. Excellent. No line to buy Octopus card or to refund it at conclusion of trip.