Mon Nedarim 85

Rava says that when the owner Assurs pleasure to all Kohanim and Leviyim, since there is no one he can give it to, he has nothing he can do with it, so it becomes hefker and the Kohanim and Leviyim may take it themselves.

The Rashba says: therefore, if someone Assurs his fruits on himself, so it's Hefker and anyone may take them. However, if he goes to a Chachum who Matir the Neder, since retroactively it was never Assur to him, so retroactively it was never Hefker. So the one who took it needs to pay for them. Even though here we allow the Kohanim and Leviyam to take it and we're not worried that they're stealing perhaps the owner will have a Chachum Matir his Neder, since it's not common to have a Neder Matir.

The proof that we're not concerned that he'll Matir a Neder hat we allow a woman who received Kiddushin on condition that she doesn't have any Nedarim on her to marry others (if she has Nedarim.) Why are we not afraid that she'll Matir the Neder and then retroactively she was a woman without Nedarim and the first Kiddushin takes effect, and now the second husband is living with a married woman. So we must conclude that it's very unlikely to Matir a Neder, so we don't worry it may happen.

The Ran disagrees. He holds that it's common to Matir Neder. We encourage people to Matir Neder, and for this reason we consider it a Davar Sheyesh Lo Matirim, since he really should be Matir it. Over there by the woman, we never allowed her to remarry. We only say that she's not Mikudeshes, since that's her status in the meanwhile. Even if we say that it is a licence for her to marry, that's only because we know she won't be Matir it forever and get herself in a lot of hot water by making her retroactively a married woman. However, when this reason doesn't apply we need to worry someone will Matir Neder.

So the Ran says, once he made a Neder and at this point he made it Hefker, so the Hefker can never be reversed, even if he's Matir Neder.

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