It's a pretty standard law. As far as the validity of it, if HiFiMan ever found out their error, they can legally request it back. If the item is not returned, they can charge the remaining balance to the buyer. They are legally entitled to the difference.
Edit P.S. This is not to be confused with the "item I didn't order." If you did not order an item, but received one, then it's a gift. However, you did order an item, but received the wrong item. This is very different and is most definitely not a gift. You legally owe the balance if you decide to keep it.
Really? Because I have been sent the wrong item many times by many different companys and they all charged me for shipping, and restocking BOTH WAYS.