Unfortunately it is not possible to return or exchange a code once you have received it. Think of top up codes as comparable with scratch cards; you would not be able to return those once they have been opened, would you? Likewise, it is not possible for us to verify if the code you wish to return has been used or not. We are unable to import the code again in our system and sell it to other customers.
Tip: You can sell/give your top-up code to someone else. Or buy a separate SIM card from the provider concerned. That way you will still be able to use the purchased top-up. Please note that your phone must be simlock free if you want to use any given provider. In many cases, codes are valid for up to 1.5 years after the date of purchase (provider related).