I have an extra permit for Monday August 4. It's a day hike only and I have no idea what the Park Service protocol is for this.
Protocol: Most people just show up and say they don't need the extra, so it becomes an "unused" permit available for anyone walking in after you.
But the classier thing to do is: go online and release your unneeded permit, making it available to people checking online for an extra slot. Someone can then get it and be sure they have a permit for a short-notice trip. Or someone needing an extra slot can pick it up.
When you release a slot online, it becomes available sometime within the next 24 hours. (So hackers can't get permits, then release them while someone is on the phone and online, watching for the permit to become available.)
I just wish Recreation.gov would reimburse people part of the $15 for releasing them online.