Unfortunately it is not possible to return or exchange a refill once you have received it. Think of refill codes as scratch cards; once they’ve been opened they cannot be returned.
Likewise, it is impossible for us to verify whether the code you wish to return has been used.
Tip: You can sell/give your top-up code to someone else or buy a separate SIM card from the provider concerned.
Please note that your phone must be simlock free if you want to use several different providers.
In most cases, refills are valid for up to 1.5 years after the date of purchase.