Since you don't have to move, unless you agreed in writing that you would move out should the owner sell it, you are in the driver's seat. If you cannot negotiate a reasonable settlement to help you move out and into another place, then you could just stay until your lease is up.
How much is always a sticky question, since people have different feelings about what is fair. If your monetary cost to move earlier than you planned is $500, then you should ask for at least that. The inconvenience to you is worth something as well.
Try to find another place and see what it would cost to move. If the new equivalent place is higher in rent, then add that to the moving cost. Add some for your inconvenience, and then go to the owner to ask if he will pay that to get you moved out right away. Don't forget you have a deposit with him and one to make on the new place. Ask if you can get your deposit back in anticipation of moving, too. He might do that, too.