If he is not the owner and doea not have a POA, he can't sell the house. You also can't force the mother to sell her home because of promises he made or agreements he entered into, since he had no right or authority to do those things. You can sue him to get back any deposit you've provided and recover compensation for any costs, losses, expenses you have or reasonably will incur due to his fraud (his lying about material, or important, things)...and you could also report him to the police and look to press charges.