Nothing to stop the OP from using the code after he receives the bitcoins. The only way to prevent that is to use the code before you send the bitcoins, but then you are basically selling coins instead of loaning them.
True, which is why I offered to let him convert it into Microsoft Points via their website, and send me the code for those upon repayment. This way, I can't reverse it, and he still has something that can easily be resold.