Round-trip tickets are significantly cheaper than one-ways TATL (usually), so appears you are coming up against a one-way fare for remaining value of your ticket. There's nothing "special" about this - not sure what agent was trying to communicate - you can go online and try to book a one-way and you'll probably get the same result. You might want to check price of another r.t. and just not do the return (if it's significantly cheaper).