It is a real snakey thing they did. I don't know if you have News/Media involvement in situations like this, but I would threaten them with media attention. I would stand outside their store with all my good friends and protest. After I got what I wanted, I would never, ever, ever, shop there again. I hate being tricked and defrauded.
If you had reserved the TV on the quoted price, they have no right of changing the price. However, if you had not made a down payment, then you have no legal tie to them as the product was not legally sold. Therefore, it would have been better if you made a down payment to secure the TV.