Arachin, Chapter 7, Mishnah 4. Our chapter has been following a sdeh achuza, an inherited family field, from the moment its owner declared it hekdesh. Mishnah 3 dealt with an owner who never bought his field back while an outsider did redeem it from hekdesh. There the arrival of yovel neither restored the land to the family that had owned it nor confirmed it in the hands of the one who redeemed it: the field returned to hekdesh, and from there it was divided among the kohanim of the mishmar on duty in the Beit Hamikdash at that hour. Our mishnah takes up the one case still left open: not a single person ever redeemed the field, and yovel has now arrived with the land still held by hekdesh.
The mishnah opens with that case: "Higia hayovel velo nig'ala", yovel has come and no redeemer ever appeared. What follows? "Hakohanim nichnasin letocha", the land is taken over by the kohanim, that is, by whichever mishmar is serving in the Beit Hamikdash at the time, "venotnim et dameha", and they hand over its value before the land is truly theirs.
Why must they pay? Because nothing has yet released this field from its sanctity. It was consecrated and never redeemed, so the kedushah is fully intact, and the only mechanism that removes such kedushah is redemption. The kohanim therefore pay the fixed sum that the Torah sets for a sdeh achuza: fifty shekalim for every beit kor. They are under no obligation to do this; no one forces a kohen to buy the field. But it is a wise investment, because once they redeem it, the field belongs to them permanently, exactly as though it had been their own ancestral field from the start. "Divrei Rabbi Yehuda" - this is the position of Rabbi Yehuda.
"Nichnasin aval lo notnin" is Rabbi Shimon's ruling: the kohanim move in and take the field, and they hand over nothing whatever. Not a single shekel of redemption money is owed to hekdesh, since as Rabbi Shimon reads the matter the Torah awards them this land while its kedushah still rests upon it. Each of these two tannaim anchors his ruling in the verses.
A third opinion belongs to Rabbi Eliezer: "Lo nichnasin velo notnin". When yovel arrives upon a consecrated field that no redeemer ever touched, the kohanim take nothing and pay nothing. As he understands the halachah, such land simply never becomes theirs. The transfer to the kohanim applies exclusively to land that was dedicated and afterward bought back by an outsider while yovel was still approaching; land that sat unredeemed the whole while is excluded.
Then what is its status? "Ela nikret sdei retushin": it carries the title of an abandoned field, staying with hekdesh and holding its kedushah "ad hayovel hasheini", until the next yovel arrives. Should anyone redeem it during those fifty years, the kohanim will receive it when that second yovel comes around.
And if no one steps forward? "Higia hayovel hasheini velo nig'ala": fifty years later the second yovel arrives and the field is still unredeemed. Now "nikret retushei retushin", it earns the name of a doubly abandoned field, and the waiting continues "ad hayovel hashlishi", until the third yovel, a full century after the first, to see whether a redeemer will finally turn up. If one does, the land is handed to the kohanim when that yovel arrives.
The mishnah ends with a general rule in Rabbi Eliezer's view: "Le'olam ein hakohanim nichnasin letocha", the field is never acquired by the kohanim, "ad sheyigalena acher", unless someone else has redeemed it while a yovel was still coming. How many yovel cycles come and go makes no difference; the ruling stays exactly the same. The land remains with hekdesh indefinitely, and it reaches the kohanim only at the yovel that follows an actual redemption.