I want to book a one-way flight from QatarAirways from Tehran(IKA) to Montreal(YUL) and the price is 1100$ for one-way trip but the price of a round-trip from YUL to IKA (YUL to IKA as outbound ~450$ and IKA to YUL as inbound ~350) is 800$ If I notify the airline that I can not join the first leg, and I was not in YUL at all, could I use the inbound flight only? I do not want to refund the first even.
If I miss the outbound in round trip booking, do I use the inbound ticket?
No. The airline will cancel your itinerary your ticket will be void.
Airlines have bizarre pricing policies and don't want you to take advatange of them. Google "hidden city ticketing" for a different version of this.
If you call them up you will either have the option to cancel the ticket or you can change your existing reservation into a one way. Qatar will charge you the price difference between the new and the old itinerary ($300) plus any applicable change fees, which depend on the type of ticket and fare rules of your original booking. You can't save any money this way and its quite likely you will have to pay significantly more.
Unfortunately not showing up - even if the airline tells you - results in cancelation of the entire ticket. There must be an automated system to do this as it happened to me twice. Even when the airline offered all passengers to skip the flight as we were on an 1-hour flight that was delayed 14 hours, meaning we could drive the distance in 5h. When it came to check in for the return, I found out that the ticket was cancelled! Luckily, they restored it but it was their own pilot that told everyone they could skip the flight.
If you call the airline to tell them that you do not indent to take the first leg, then they will rebook you which is not the same as a no-show. You will be charged the difference in fare which cancels out the price advantage, plus a change-fee, depending in conditions when purchasing the ticket.
You are better off buying the ticket the other way with the first leg in the direction you will be flying. Airlines do not like when people miss the second half but it least your ticket will not be cancelled and they are unlikely to cause an issue if this is a rare occurrence. Keep in mind though, they they can deny you future service if they choose.
Even if you are not planning to return, you can place the return segment far off in the future away from any high season to get good pricing. Plus, for a fee, you will generally be able to change that leg or postpone it again until a time when you will need a flight.