accordion to rebbi yossi, the gemara wants to say that after 3 days the מפקיר can be חוזר but m'drobonon they said he cannot. still, as the ran explains, the חזרה does work to the extent that the זוכה is obliged in giving תרומות ומעשרות מדאורייתא.
in other words, there is no הפקר ב"ד going on here because then the זוכה would not have to give trumos umaasros, still he is allowed to keep the hefker! why? isn't he stealing?
(as an aside, very worth while to look into the תורת זרעים on פאה פ"ה מ"א regarding hefker and our sugya)