Commentaire sur Halla 4:4

יטול מן האמצע – from the place that they adhere one to the other, we find that we separate [Hallah] from both of them.

A kav of [dough made from] new grain and a kav of [dough from] old grain which are stuck together: Rabbi Ishmael says: let him take [hallah] from the middle; But the sages prohibit. Here there are two batches of dough, each of which consists of a kav, less than the minimum measure to make it liable for hallah. One batch is from new grain (lesser quality) and one is from old grain (greater quality). The two batches are stuck together, thereby making them liable for hallah according to all opinions. The question is whether one can take hallah from one in order to exempt the other. Rabbi Ishmael says that one should take from the middle of the two batches, and in this way he will take from both and exempt the entire batch. However, the sages prohibit and say that he must take from the old to exempt the old and from the new to exempt the new. Hallah that he separates from the new cannot exempt the old, nor can hallah from the old exempt the new, even though they are from the same species. Note that these two batches are similar enough to join together but not similar to allow one to separate from one for the other.

וחכמים אוסרים – for one who sees that it is thought that it is permitted to give priest’s due and to tithe from the new grain [of this year] with the old grain [of last year] and from the old grain [of last year] with the new grain [of this year]. And the Halakha is according to the Sages.

One who has taken hallah from [dough made out of] one kav: Rabbi Akiva says: it is hallah; But the sages say: it is not hallah. If one takes hallah out of a batch of dough that is not liable for hallah, Rabbi Akiva says that that which he takes out is considered hallah and all of the rules of hallah apply to it. Since he called it hallah, it is hallah. Rabbi Akiva seems to think that what makes hallah is the person, not the dough. The other sages say that it is not hallah, because if the dough wasn’t subject to hallah, that which he takes out of it cannot be hallah.

מן הקב – which lacks the [appropriate] measure for Hallah (i.e., five-fourths).

ר"ע אומר חלה – and he that completed the dough afterwards according to the [appropriate] measure for Hallah, for since the measure was completed, it is [liable] for Hallah retroactively.

אינה חלה – since at the time when he separated it, the dough was exempt. And the Halakha is according to the Sages.