Tues Nedarim 44

The Gemara tries to reconcile the beginning and the end of the braisa. The Raisha seems to say that once he made it Hefker, it leaves the owner's possession immediately, like Rabanan. The Seifa, where he made a time limit for when it will be Hefker, implies that he may renege on his Hefker, so it doesn't leave his possession until someone else takes it.

The first answer that it's like the Rabanan, however the Seifa is different, since it's not common to make such a Hefker.

The Ran explains: since he did something uncommon and did not do the regular type of hefker, forever, we see he's reluctant to mafkir it. Therefore, we assume his intent is not to release it from his possession until someone takes it. However, most people by regular Hefkir is not reluctant, so they release it from their possessions immediately.