Nedarim 44: Hefker: Vowing that Property is Ownerless

If a person vows that a field will be hefker, ownerless, for a day, a week, a month, a year, or a Sabbatical cycle (seven years), that decision is valid and cannot be retracted.  Or can it?  It might depend on when one took possession of the field.  If no-one took possession of the field, the decision can be retracted.  The rabbis are concerned that people might forget their vows over time.   As well, they are aware that "swindlers" might declare a field ownerless to avoid tithing and then reclaim the field.  The Gemara goes on to describe different produce and the halachot of pe'a.

More and more, I am understanding Nedarim as a masechet about those times that we say, "If you just do this thing for me, I swear I'll leave you alone."  People makes promises that we just don't keep.  The rabbis want to ensure that we take our vows seriously.  In the language of halacha, that means determining the specific rules and parameters we put on our vows.