A court will hold you and the seller to what you agreed to and what is stated on the written documents you signed at the closing. What you may or may not have been told would be very hard to prove in court and would likely cost more tha putting in AC would--or maybe close to the same price.
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If the sales doc contract listed central ac, it probably would be worth pursuing with an attorney as the ac and duct work could run 6-10, 000. But if it is only something that was said, I agree with the other attorney.