The solution is to buy your tickets from a travel agent or from a hotel or from some other agent. You'll pay a bit more, but they'll arrange for the purchase of your return ticket, which you pick up at your destination. We didn't figure this out until it was too late, so we didn't go to Datong. Which was just as good for us - we were short on time and too tired to go anyway). We were both greatly relieved once we decided not to go.