An overnight pass is fine for a dayhike. Just be sure to check in (telephone is ok) by noon the day before the hike.
Edit... But just now I notice you are dayhiking on the day AFTER your overnight entry date. THAT will probably cause a problem, since in the permit system, the ENTRY DATE is the key. You should call the ranger station. Good luck.
As for letting your unused permits go, be nice to everyone else, and call in as soon as possible and release them so others might reserve one.
Here is another recent thread on releasing unused permits:
Solicitation for Mt. Whitney Summit Trip partners