OP, have you considered the receiver may just be claiming he didn't get the coin? To get you to send again?
He got super scammed, clearly. The "receiver" sent him a BTC address, then went "Oops it was the wrong one." *roll eyes*
Please, common.
I too guess the possibility of second one.
Because many people are using cold storage wallet addresses and many bitcoin address. So, watching a address is possible only with a expected transaction. Other wise it may take a year too for the receiver to find that wrongly sent amount. At that time he may not recognize it as wrongly received.
This must be a very sad situations if we are unable to contact them through signing a message with another low amount of transaction.
Any way sending a message through a transaction will make him to react even after a year also would be fine. The catch is he must be royal enough.