One way, I know of, that it can be done is to open a free account with pirateship (PS). You can then create a return label easily (they have good instructions how to do that, they also respond to messages promptly so you can ask them how to do it). Then you can send a link to the 'buyer' or attach the image of the label.
Another option, which I did in the past, was to ask the 'buyer' if they could mail it directly to the actual buyer. This may get complicated, if they opened the package already and would have to repack it. But your 'buyer ' seems to be a nice person, he could do it. With PS it shouldn't be a problem to generate a label addressed to the actual buyer with the sender being your 'buyer'.
Lastly, if they did not open the package they can just write "return to sender" and it will be returned to you. However, this may take a long time, so your actual buyer may get antsy. You need to keep your actual buyer posted about what you are doing.
