Explain clearly that you wish to cancel only your return flight, not the full itinerary. The Qatar representative will check your fare conditions, as cancellation rules vary by ticket type. Some tickets are non-refundable, while others offer partial refunds or travel credit. If your ticket includes flexible return options, you might even be able to reschedule instead of fully canceling.
Canceling in advance also ensures your frequent flyer miles, if used, are either returned or adjusted according to airline policies. The agent may also waive certain change fees if your reason involves illness, schedule conflicts, or emergencies. Always request a confirmation email that reflects the updated booking status, especially if the outbound flight has already been flown.