The gemara (Bechoros 12b) writes that if you trade in peiros shevi’s for a cow, the calf of this “beheimas shevi’s” would be exempt from dinim of bechora. Since the kedushas shevi’is of the fruits transfer to the cow, it’s bechor cannot be offered as a korban because peiros shevi’s are “l’achla,” only for eating, not for being burned. The Rambam offers his own reason for this din (maybe based on a different girsa – see the nosei keilim) and quotes the derasha that peiros shevi’is are only “l’achla” to the exclusion of “sechora,” business transactions.
Asks the gemara: by this same logic, why don’t we say that the mitzvah of challah should not apply during shemitah? Since there would be a chiyuv to burn challah if the dough became tamei, why not apply the same rule of “l’achlah” but not for burning? (How the Rambam would learn this question if the reason of beheimas shevi’is has to do with sechorah is difficult).
The gemara answers that there is a special gezeiras hakasuv of “l’doroseichem” by challah that tells us that it applies at all times.
The Maharit Algazi is not happy with this question of the gemara. His objection is built on three assumptions, the first two being relatively simply, the third being quite a chiddush:
1)There is no shiur m’doraysa as to how much dough you need to separate to fulfill the mitzvah of hafrashas challah. Just like separating one stalk of wheat is enough to fulfill the mitzvah of terumah m’doraysa, so too, you can seperate a drop of dough to fulfill the mitzvah of challah.
2) The prohibitions of burning or sechorah stem from an aseh of “lachlah.” Since the shiur achilah in the Torah is always a k’zayis, the shiur of these issurei aseh must also be a k’zayis.
3) The din of chatzi shiur only applies only to a lav and not a mitzvas aseh. Therefore, there is no issur of chatzi shiur on an issur aseh either.
Putting it all together, he asks: why is the gemara bothered by how to be mafrish challah during shemitah – just be mafrish less than a k’zayis, which is enough to fulfill the mitzvah d’oraysa of challah but less than the shiur of the issur aseh learned from “l’achlah?”
Is assumption #3 really true? We’ve discussed before whether there is any significance to chatzi shiur of a mitzvas aseh, but even if one holds there isn’t, maybe one can still argue that the din of chatzi shiur should still apply to an issur aseh since it effectively functions like a lav.