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Towards the latter half of masechet Erchin we learn about important differences between a sde achuza and a sde mikneh. A sde achuza is one’s ancestral land. These lands cannot be sold forever and instead return to the original owners in the yovel (jubilee) year. We learnt however, that if the owner is makdish (consecrates) the land and does not redeem it before the yovel year and/or it is sold to another party, then it does not revert back to the original owners. Instead it is divided amongst the kohanim that are serving when yovel begins.
A sde mikneh on the other hand, refers to purchased land that was not one’s sde achuza. There are a number of differences between the two types of land. One difference mentioned in the Mishnah (7:5) is that if one consecrates a sde mikneh and it is not redeemed prior to the yovel, it nevertheless returns to the original owner. The Mishnah explains, “One cannot not consecrate something that does not belong to them”. This is a general principle that one cannot take another’s property and consecrate it. In this case, since the purchased land is effectively a lease until the yovel year, the land is not the purchaser’s to dedicate to hekdesh perpetually.
The Tosfot Yom Tov notes that the rationale provided by the Mishnah may appear unnecessary. When teaching about the case where one consecrate a sde mikneh the pasuk itself states explicitly that (Vayikra 27:24): “In the Jubilee Year the field shall return to the one from whom he acquired it; whose ancestral heritage of the land it was.” The Tosfot Yom Tov however explains that the pasuk is mentioned in the context of where the person who consecrated the sde mikneh subsequently redeemed it. The Mishnah was therefore required to teach that the law also applies if a third party or no one redeemed it prior to the yovel.
A question brought down in the name of the Griz (Stencil 14a) is why the rational of the Mishnah was required at all. The fact that consecrated land does not return to the owners is a novel law that is only mention regarding sde achuza. In other words it is one of the laws of sde achuza, so why should one assume that it applies elsewhere? The Griz understands therefore that this law regarding consecrated land not returning to the owners is not a law in sde achuza. Rather it is a law in yovel. Consequently he understand that if there was a situation where there would not be an issue of consecrating property that belongs to someone else for a sde mikdneh, then it would, just like a sde achuza, be divided amongst the kohenim in the yovel year.
The Minchat Chinuch (355:5) raises such a case: if one sold their land ten years before the yovel for sixty years. The Gemara (Bava Metzia 79a) teaches that the land would not return to the owner until the second yovel. Consequently there would be no issue of “ein adam makdish davar she’eino shelo”. Nevertheless that Minchat Chinnuch argues that the core reason is not “ein adam…” but rather the above mentioned pasuk. He understands that the Mishnah’s comment of “ein adam…” is simply because the logic applies in most situations.
The Kehillot Yaakov (7)1 writes however that both reasons, the pasuk and general rule, are required. Had the law just been based on the pasuk, one might think that it would only return to the owners without to provide monetary redemption, if it had already been redeemed. Otherwise, one might think that the sanctity cannot simply disappear when the yovel arrives. Consequently, the rule of “ein adam…” teaches that hekdesh never had control over it for the yovel year and it can be returned to the owner without redemption.2
1 Cited in a footnote in the Yalkut Biurim, Erchin 26b (Metivta)
2 Whether the sde mikneh can return to the owners without having ever been redeemed is the subject of debate. See the Tosfot Yom Tov.