Buy the cheapest roundtrip ticket you can get from your preferred airline. If you can't make your return flight, call the airline and cancel the flight. Explain the situation and tell them you will call them back when you know when you can return. It usually costs about $250 to make the change, depending on the airline, but ask what the rules are before you buy the roundtrip ticket. Some airlines require that you book the new return flight at the same time that you cancel the original return while some will allow you to wait until you are sure.