You don't say that you are in England and Wales, or whether you have exchanged contracts. If you have, then there is an obligation to sell the house to you at that price or (at the worst case) a route to compensate you if they fail to complete the deal. If you haven't exchanged yet, either party are at liberty to walk away from the deal, so yes, this could happen. I imagine it has been discovered that the selling price is less than the mortgage sum plus any loans secured on the house (second legal charges). It seems rather late for the seller's mortgage company to find that out