I think you may be stuck. I work part time for a store that sell those. When the cards are sold, they are activated through the register via the barcode on the back even though the code isn't revealed until you scratch it off. I found this in the fine print on the iTunes site (where you buy gift cards);
"Prepaid cards are not redeemable for cash and cannot be returned for a cash refund, exchanged, or used to purchase gift certificates or provide allowances."
You may be able to resell it on eBay or something if the code on the back isn't revealed (that'd be one way for a buyer to know it hasn't been used and still has credit on it).