I agree take it back with your receipt. In addition, if you don't have your receipt it should be in the computer that it was purchased and not stolen. Be firm with the manager and insist that he looks that air conditioner up. A similar situation happened with me at Target with a TV. I didn't have the receipt but I insisted that they look it up in the computer and I was refunded my money.
Shouldn't be a problem with receipt
Take the receipt and get another one.
They might have an expiration date to when u can return it
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