RETRACTION OF HEFKER
(Beraisa #1): If Reuven declared his field to be Hefker, for three days he may retract. After this, he cannot retract. (The Rosh holds that someone else can acquire it before Reuven retracts. The Ran disagrees.)
If he said 'this field should be Hefker for one day', or week, month, year or Shemitah cycle, if no one has yet acquired the field, he can retract;
Once someone acquired the field, he cannot retract.
Question: The Reisha is like Chachamim, and the Seifa is like R. Yosi (like R. Yochanan said above, that R. Yosi allows retraction from Hefker until someone takes it.)
Answer #1 (Ula): Also the Seifa is like Chachamim.
Question: If so, why can he retract before someone acquired it?
Answer: Hefker for a year or Shemitah cycle is different, for this is not common. (Chachamim agree that he can retract before someone acquired it. It seems that he did not really want someone else to acquire it.)
Answer #2 (Reish Lakish): Since the Seifa is like R. Yosi, also the Reisha is like R. Yosi.
In the Reisha he can retract only for three days, lest the law of Hefker be forgotten. (If he could always retract, it seems that it was not Hefker and must be tithed. Perhaps people will tithe from it, which is really exempt, to exempt real Tevel!)
Question: If so, even the first day, he should not be able to retract!
Answer (Rabah): Chachamim did not say so, due to swindlers who declare their fields Hefker (to exempt them from tithes) and take them back again. (Since we allow them to retract, it is not Hefker, and tithes must be taken.)
Inference: Reish Lakish holds that mid'Oraisa, the field is not Hefker at all.
Question: If so, perhaps he will come to tithe from produce that must be tithed (i.e. mid'Oraisa it is not Hefker) on what is exempt (true Hefker), or vice-versa!
Answer: We inform him that he must tithe the produce of this field from this field itself.
Question (Beraisa #2): If one made his vineyard Hefker and promptly harvested it, he must leave the usual gifts for the poor: Peret (grapes that drop during harvesting), Olelos (deficient clusters), Shichechah (grapes forgotten during harvesting), and Pe'ah (a corner of the vineyard unharvested. Hefker is exempt from these, but "abandon" is repeated to obligate when he re-acquired it.)
He is exempt from Ma'aser.
We understand according to Ula. The Beraisa is like Chachamim. It teaches that the vineyard is Hefker mid'Oraisa (but mid'Rabanan, he must take Ma'aser.)
But according to Reish Lakish, why is it exempt from Ma'aser?
Answer #1: Reish Lakish explained Beraisa #1 like R. Yosi. He admits that Beraisa #2 is like Chachamim.