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If someone is Mafkir his field for a day, a week, or a month he may back out as long as no one was Zocheh in it yet. (1)
Reish Lakish says that if someone is Mafkir something he may only back out the first three days according to R. Yosi. (2)
R. Yosi was concerned for the Rama'im and therefore for the first three days the Mafkir may back out. (3)
If someone is Mafkir his field and after three days someone is Zocheh in it, we tell him to separate the Ma'aser from the produce of the field itself according to R. Yosi. (4)
If someone is Mafkir his vineyard and the next morning he harvests it, he is Chayav in Peret, Olelos, Shichechah and Pe'ah, but he is Patur from Ma'aser. (5)
According to R. Yosi if someone is Mafkir his vineyard and the next morning he harvests it, he is Chayav in Ma'aser since it never left his possession.
A BIT MORE
1. The Hefker does not leave the possession of the original owner until someone is Zocheh in it therefore the original owner may back out. According to Ula even the Rabanan agree with R. Yosi that when someone is Mafkir something for a fixed time it does not leave his possession until someone is Zocheh in it.
2. So that we will not forget the Dinim of Hefker. If the Mafkir was allowed to back out the recipient might think that he received it as a gift and it is Chayav in Ma'aser.
3. The Rama'im are Mafkir their fields because they want to be Patur from Ma'aser and they intend to immediately be Zocheh it back from Hefker themselves. Therefore the Rabanan said that for the first three days it is not Hefker and it is Chayav in Ma'aser if they take it back.
4. The produce from this field is Chayav in Ma'aser mid'Oraisa according to R. Yosi since mid'Oraisa he had the right to back out. Even though the Rabanan said he cannot back out after three days, since he has the right to back out it lost its status as Hefker and therefore it is regarded as a gift which is obligated in Ma'aser. However we are concerned that the recipient will think that it is only Chayav in Ma'aser mid'Rabanan and will separate Ma'aser from something which is Chayav in Ma'aser mid'Rabanan to be Motzi this produce from Ma'aser or vice versa, therefore we tell him to separate the Ma'aser from the produce of the field itself.
5. Even though Hefker is Patur from Matnos Aniyim since he took it back himself from Hefker it is Chayav in all of the Matnos Aniyim.
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