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Unfortunately, if you signed an extension you are bound to the terms of that extension you signed. The reason people use written contractual agreements is because they intend to force people to honor agreements they make. So you will be bound to the terms of the extension contract you signed. If you do not honor that contract, you are in breach of the contract. If you are in breach of contract, he legally is correct that he can sue you for rent until he rents to a new tenant as well, since you left before the extension was over and you signed the extension I am afraid.
Had you not signed the extension, you had a potential way to cancel the lease based on his failure to disclose before you signed the lease that he was planning on selling and that allowing open houses during your lease interfered with your peaceable use and enjoyment of the premises that the lease guaranteed you. However, when you signed that extension, you knew he was planning to sell and unfortunately, that legally committed you for the term of the extension and changed the terms of the original contract and you are now bound by that extension contract.