Consumer Law, Warranties engine trouble

[mage16]

I purchased a used engine on Sunday for a Nisan quest. I was told I was getting a quest motor with mileage between 80 and 90 thousand miles on it. When I checked the stock number with their online inventory, I found that they had sold me an engine from a mercury villager with more than double the stated miles on it. These 2 cars have the same engine. When I called back and mentioned that they miss-represented the engine, the online inventory changed within minutes to no longer show any information on the villager engine. That first told us their system shows that we had the quest engine, but when we confirmed the stock number on the engine and invoice, they told us that they want another $250 for the engine they told me I was getting. Now they won't refund my money or exchange the engine. Is there anything I can do?