Nedarim 44A

אמר ״תהא שדה זו מופקרת ליום אחד״, ״לשבת אחת״, ״לחדש אחד״, ״לשנה אחת״, ״לשבוע אחת״⁠ — עד שלא זכה בה, בין הוא בין אחר יכול לחזור בו. משזכה בה ⁠— בין הוא בין אחר, אין יכול לחזור בו. רישא רבנן, סיפא רבי יוסי!

If one said: This field will be ownerless for one day, for one week, for one month, for one year, or for 8-year Sabbatical cycle, as long as no one took possession of the field, neither the one who declared it ownerless nor another person, he is able to retract his declaration. Once one took possession of the field, whether it is he or whether it is another person, he is unable to retract his declaration. This baraita is difficult, as the first clause is apparently in accordance with the opinion of the Rabbis, who hold that rendering property ownerless is unilateral and not contingent upon whether one took possession of it, and the latter clause is in accordance with the opinion of R' Yosei, who holds that rendering property ownerless is complete only when one takes possession of that property.

אמר עולא: סיפא נמי רבנן היא. אי הכי, אמאי עד שלא זכה בה בין הוא בין אחר, יכול לחזור בו? שאני שנה ושבוע דלא שכיחי.

Ulla said: The latter clause is also in accordance with the opinion of the Rabbis. The Talmud asks: If so, why is the ruling in the baraita: As long as one did not take possession of the field, neither the one who declared it ownerless nor another person, he is able to retract his declaration; according to the Rabbis, once he declared the property ownerless, although he can take possession of it, he is unable to retract the declaration. The Talmud answers: It is different when one declares an item ownerless for a year or a 7-year period, as doing so is uncommon. Since from the outset he limited the duration of ownerless status, clearly he reserved for himself certain rights; therefore, he can retract his declaration.