It would seem to me that is YOU who are in the position of power. If the landlord truly wants to sell next year then it would be a hassle for them to rent out the place should you move out, unless short term rentals are common in your area. They'd have to pay the agent a fee even if they found someone who wanted to rent short-term (although there's no reason they have to tell the next person they are selling I guess).
I would use this to your advantage and not be afraid to push hard to get favourable terms.
Have you told the EA you plan on moving anyway due to DH's change of job? If you haven't, please DON'T! The EA will then know you are in a tight spot too, better they think you can easily sign a long-term lease in another house next door without the hassle of it being sold from under you.
