I'd agree with Johan that this sounds extremely dubious. If I were you, I would call again and ask confirmation from another agent. If you get the same answer, then make sure that you keep the details of the names of the agents and the date and time of conversation, so that you have something to come back with if, I I suspect will happen, you do not get credited 100% on both legs. If, as is more likely, the second agent tells you that what you had been told was not right, then ask for your booking to be changed to a 100% mileage earning fare for the return leg (and make sure that they waive any change fee that might exist for doing so).