Bill them for the time out there, and if you don't all ready have it implement a early cancellation clause. For example if they cancel for no reason other then to give joblow the work, they have to pay x amount.
Did they SHE your customer contact you stating a CANCEL. you could be on the hook if there is a slip and fall or Jobow didn't show up for 12"+ snow fall, because you are still the guy she hired. If you want to cancel her get it in writing.
Built Ford Tough