Received message Just a heads up that [guest] made other arrangements for Sep 9 - 10, 2016"

[Andrew45]

Despite getting this message hours ago, the reservation is not cancelled, so the calendar is not open to re-book.  Also, since it is not marked "Cancelled" I don't know if I can safely mark it as available on other websites.  

[Gerry-And-Rashid0]

Who sent you the message? It does not sound like an AirBnB message - they wouldn't say...Head up..etc..

Message your guest and remind them that if they do not cancel their booking they will pay the full amount as the room remains booked for them.

If they are hoping that you will cancel the reservation - then as you know, you should not, as you will be penalised.

Right now you have a paid for booking, so it would be unwise to offer those dates on other sites just in case you guest changes his mind and decides to stay and not cancel.