Arachin, Chapter 7, Mishnah 5. The chapter has so far dealt with a person who sanctified his sdei achuzah, the field that came to him as his family's inheritance in Eretz Yisrael. The Mishnah now takes up a second scenario: a field acquired by purchase from another person, which the buyer then declared Hekdesh.
The starting point is the nature of such a purchase. A buyer of an ancestral field never acquires it permanently; he holds it only until the Yovel year, at which point it reverts to the family that originally owned it. It follows that if the buyer consecrates the field, his dedication reaches no further than his own rights: it lasts until Yovel and no longer, and then the field returns to its original owner.
When the Seller Is His Own Father
Here the Mishnah presents a son who acquired an ancestral field from his own father and declared it Hekdesh. The father then died while Yovel was still in the future, so that the son now holds ancestral rights in this very field as his father's heir. What happens once Yovel arrives? Does the land come back to the son, whose ancestral claim to it is now established, or does it go over to the kohanim, the fate of a sanctified ancestral field left unredeemed?
"Halokeiach sadeh mei'aviv": one who buys a field from his father. "Meis aviv": the father then died, "ve'achar kach hekdishah", and only after that death did the son declare the field Hekdesh. "Harei hi kisdei achuzah": the field carries the status of an inherited field rather than a bought one. The son's first connection to this land was indeed through a purchase, but at the moment he sanctified it his father was no longer alive and the field had already come to him by inheritance, with full ancestral standing. His declaration was thus made upon an ancestral field, and every halachah of a sanctified sdei achuzah applies to it.
"Hekdishah": if, however, he sanctified the field first, "ve'achar kach meis aviv", and his father passed away only later, then "harei hi kisdei miknah", the field keeps the status of a bought field. At the moment of sanctification he possessed nothing beyond the limited rights of a purchaser, so what he sanctified was a purchased field. "Divrei Rebbi Meir", such is Rebbi Meir's ruling. In practice the field is then redeemed according to its market value, and Yovel does not deliver it to the kohanim.
Follow the logic through. Since at the time of dedication the son's hold on the field was temporary and he was obligated to return it to his father at Yovel, he stands like any other purchaser who consecrates a field he bought. When Yovel arrives, the field automatically reverts to the father. The father is no longer alive, so the field passes through him to his heir, and in the end it reaches the son.
A Second View: Rebbi Yehudah and Rebbi Shimon
"Rebbi Yehudah veRebbi Shimon omrim: kisdei achuzah." These two Tannaim hold that even when the son consecrated the field while his father was still alive, it is treated as an ancestral field. It is redeemed at the fixed Torah rate of fifty shekalim for forty-nine years, and if the owner does not redeem it before Yovel, the field goes to the kohanim rather than back to him.
"Shene'emar", these Tannaim bring support from the language of the pasuk that treats the sanctification of a bought field: "ve'im et sdei miknato", if he sanctifies to Hashem a field he purchased, "asher lo", which is not, "mi sdei achuzato", of the fields of his ancestral holding. From here they read: "sadeh she'eina re'uyah", a field that has no possibility of becoming "lihyot sdei achuzah", his ancestral holding, meaning land bought from a stranger in whose family he has no share whatever. Such land alone is governed by the halachot of a sanctified purchased field. "Yatza zo shehi re'uyah", excluded is the field before us, taken from his own father, which is fully capable of becoming "lihyot sdei achuzah", his ancestral portion. Even where he sanctified it while his father was still living, the land at that moment was already property that could reach him as an inheritance, and it is therefore judged by the rules of an ancestral field.
According to Rebbi Yehudah and Rebbi Shimon, then, the sequence of events makes no difference at all. Since the son bears a relationship to the owner of the field such that in some circumstance it could become his ancestral portion, that potential is enough. A field like this can never be classified as a sdei miknah; in every case it is treated as a sdei achuzah, whatever the order of the dedication and the father's death.
The Law of a Purchased Field
The Mishnah closes the subject with the halachah of an actual purchased field. "Sdei miknah", a bought field that its purchaser sanctified, "eina yotzei al hakohanim", is not transferred to the kohanim, "baYovel", when Yovel comes; it goes back instead to the family that held it from the start. The reason: "she'ein adam makdish", nobody has the power to sanctify, "davar she'eino shelo", an item that is not his. The purchaser's ownership expires at Yovel, when he must restore the land to the family whose portion it is, and his act of sanctification is bounded by that same limit.
Kohanim and Levi'im
All of this, that a sanctified ancestral field left unredeemed is handed at Yovel to the kohanim and is gone from the original family for good, speaks of land belonging to ordinary Yisraelim. When the owner is a kohen or a levi, his holdings follow a different set of rules.
"Kohanim uLviyim makdishim le'olam": members of these two families may sanctify their fields at any point, the Yovel year included, while for an ordinary Yisrael the Chachamim learn from a pasuk that his sdei achuzah may not be sanctified in the year of Yovel itself. "Vego'alin le'olam": they may likewise redeem whatever they sanctified whenever they wish, "bein lifnei haYovel", before Yovel arrives, "bein le'achar haYovel", and after it has passed. Their fields are never turned over to the mishmar kohanim once Yovel comes. Thus a kohen or levi who sanctified his ancestral field and let Yovel go by without redeeming it may still redeem it afterward. This too the Chachamim learn from the pesukim.