I don't see how a queen size mattress is equivalent to a bigger king sized one.
Maybe I don't understand your query, and maybe you did get the king sized bed you wanted. Anyway, as long as you got the bed you wanted with the warranty you wanted, I don't think what's on the receipt matters to you.
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So, you were sold one mattress, then asked to return and sign a receipt for a smaller mattress for the same price and with a warranty agreement? Do you still have the voided receipt? Can you show what the price was, as advertised, for the king mattress? For the queen mattress?
There are a ton of questions going on here; it's possible that, yes, you have some recourse if the store refused to honor a signed contract - but you agreed to a modification, so that ship has probably sailed. It is unlikely, since you signed off on the changes, that you're going to be able to do anything. You also could have refused delivery of the mattress you didn't want, which would have entitled you to a refund, at least. But, you accepted it and that ship, too, has sailed. You could speak to a local attorney and bring in all of the paperwork, but the sad truth is that you have probably waited too long to do anything except try to work out a return with the store.
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