Nedarim, Chapter Eleven, Mishnah Three
The first section of the mishnah continues to discuss which vows a husband may annul. The second section of the mishnah discusses a person who swears that priests and Levites may not derive benefit from him.
1) [If she vows], Konam, that which I benefit from mankind, he cannot annul, and she can benefit from gleanings, forgotten sheaves, and the corners of the field.
2) [If one says], Konam be the benefit which priests and Levites have from me, they can take [from him] against his will.
3) [But if he vows,] Konam be the benefit these priests and Levites have from me, other [priests and Levites] should take.
Section one: The husband cannot annul this vow because there are still people from whom she may receive benefit (food, clothing etc.). Mankind implies people other than her husband. Since her husband may still provide for her, this is not a vow of self-denial. Furthermore, she may still collect the produce which is mandated to be given to poor people from peoples fields. These do not actually belong to the owner of the field; rather they are considered ownerless. This includes gleanings which is the wheat which falls from peoples hands as they collect, forgotten sheaves, and the corners of fields.
Section two: A person must give certain parts of his produce to priests and Levites. He must give terumah to priests and first tithe to Levites. If he vows that priests and Levites should not be able to derive benefit from him, they still may take terumah and tithe, because these do not actually belong to the one who vowed.
The connection between this section and what we learned above is that just as things left for the poor do not actually belong to the owner of the field, so too terumah and tithes do not actually belong to the person who separates them to give them to the priests and Levites.
Section three: In this case the one who vows does not prohibit all priests and Levites from benefiting from him, just certain priests or Levites. In this case the vow is effective and other priests and Levites should take the terumah or tithes. Although as we stated above the person does not own these, he has the right to choose to whom to give them. Therefore the vow is effective.