If the lease is current, as far as I know it can't be broken. It wasn't fair of the current owners to do this to the tenant!
We've lived in a few houses for sale. They are usually only available for a short term lease (3 - 6 months) if they are on the market, or going on the market. Our last place was sold strictly as an investment property so we could stay in the house as long as we liked.
The 2 houses we did have to move out of were when the lease was up & not before.