You can't do Western Union recalls. In my experience and how it works, You send money through a wire. There is not binding contract on why you send the money, it's just a money forward. They don't do the whole, "You didn't give him the product" spiel. So you have to be very safe. However, On their site you can contact and cancel a wire transfer, but really if your dealing with someone who is ripping you off, your out of luck cause they probably are already there picking up the money.
They prove that they are the person who is suppose to receive the money with an id, they get paid in cash. Once it's been delivered, there is no recalling. It is there word against yours, considering this is just a agreement to transfer funds, and has nothing to do with an actual checkout process.
Hope this helps,
Scrawl