I just cancelled a reservation for a single day-use permit for July 20, so it'll probably be showing up as available at recreation.gov shortly. I hope this helps somebody!
I was a little surprised that my $21 payment included a $6 non-refundable reservation fee and a $15 penalty, for a net balance of zero. So I get nothing for canceling, other than the good feeling of helping someone else get a Whitney permit.
