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I suggest that you send them a letter by registered mail. In the letter set out that they assured you that you could get a full refund of the deposit if you changed your mind, and tell them that you are enclosing a copy of the email for reference.
Give them a deadline for refunding your deposit in full and tell them if they fail to comply you will commence legal proceedings and then will be seeking the $500, interest, and your costs.
You can then sue them in Small Claims Court and self represent. If it gets that far and you find you need some help as you go through the process you can always post on this site again and I can assist you further.