I don't know why you have to go back and sign. Did your husband read all the paperwork? If he can only qualify with your name on the loan, and you don't qualify or you choose to return the car, you may not get your $500 back. Why? Check the contract to see if this is a liquidated damage or non-refundable fee.
Your best bet to get the money back is: "you warranted/promised my husband would have this car and qualify. Now after we take possession you say you need my sig? That is fraudulent and we want our money back for the misrepresentation". If they don't budge, call your local better business bureau.
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I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.