You need the return ticket when you're leaving so you can fill in the departure immigration form, so technically you will have a return ticket when you leave Canada. Once you're down there you can go online and cancel the ticket, you will probably lose $50 off the return value for the cancellation, but the rest will be held in your name with the airline that you can use towards your return ticket when you're ready to re-purchase it. You can modify the departure slip they give you on arrival at Jamaican immigration in the airport for the new return flight once you have the information and and submit that when you're ready to leave. I've done it more than once, you should have no problems.