Gift cards are country-specific, so the ones that you can buy in France can only be used in France with French iTunes account. If you don't know anybody else in France that you could sell or give it to then you can try contacting iTunes support and see if they will refund or credit you (if they do then they are more likely to credit your iTunes account as opposed to refunding a credit card, so selling the card might be a better option if you don't use iTunes yourself) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption