WHAT RABI YOSI WILL SAY - ACCORDING TO HIS OPINION THAT HEFKER OCCURS WHEN SOMBODY ELSE OBTAIN IT - ABOUT HEFKER IN SHABAT ? ABOUT HEFKER IN SHmita?
The Chasam Sofer (43a) says that Rebbi Yosi of course admits that during Shemitah everything is Hefker even before someone else takes possesion of it, as Hash-m has proclaimed it ownerless. I am not sure what you are asking regarding Hefker on shabbos.
All the best,