1) THE "AVUNGAREI" WORKERS OF THE REISH GALUSA
OPINIONS: The Gemara relates that Rav Chama permitted the "Avungarei" of the Reish Galusa to work on Chol ha'Mo'ed. He reasoned that since they do not receive an actual wage but merely eat from the table of the Reish Galusa, they may work on Chol ha'Mo'ed.
Who were these "Avungarei," and what type of Melachah did Rav Chama permit them to do on Chol ha'Mo'ed? Were they Jews who performed a type of Melachah which is permitted on Chol ha'Mo'ed, or were they Nochrim who performed a type of Melachah which a Jew may not do on Chol ha'Mo'ed?
(a) The NIMUKEI YOSEF and RITVA (in his first explanation) explains that the Avungarei were Nochrim who performed Melachah which a Jew may not do on Chol ha'Mo'ed.
The Ritva, however, rejects this explanation because the Gemara says that "Rav Chama permitted the Avungarei to work," which implies that the Avungarei themselves asked him whether they were permitted to work. If they were non-Jewish laborers, the Gemara would have said that Rav Chama "permitted the Reish Galusa to have the Avungarei work for him"; the Avungarei themselves obviously would not have brought a Halachic inquiry before Rav Chama.
(b) The RITVA cites RABEINU YEDIDYAH who explains that the Avungarei were Jews who performed a type of Melachah which is permitted on Chol ha'Mo'ed (such as a "Ma'aseh Hedyot").
If, however, the Melachah itself is permitted, why did Rav Chama say that the Melachah would have been prohibited if the workers received a wage? Evidently, Rav Chama rules that even a type of Melachah which normally is permitted on Chol ha'Mo'ed may not be done if one is paid for doing it (in which case it constitutes an "Uvda d'Chol," a weekday activity). This is also the explanation of the ROSH.
HALACHAH: The SHULCHAN ARUCH (OC 542:1) rules like the Ritva and Rosh (in (b) above) and permits a Jew to do a type of Melachah which is permitted on Chol ha'Mo'ed (such as "Ma'aseh Hedyot" or "Tzorech Mo'ed"), but only when he does not receive a wage. If, however, the Jewish worker is very poor and has no money even for food, the Jewish employer may hire him and pay him a wage on Chol ha'Mo'ed (OC 542:2).
The REMA (OC 542:1), based on the KOL BO (see, however, BI'UR HALACHAH), permits one to pay a set wage to a Jewish worker to do a Melachah which is necessary for a Davar ha'Aved on Chol ha'Mo'ed. (The Bi'ur Halachah extends this leniency to paying a Jew to do a Melachah which is needed for Ochel Nefesh.) However, one preferably should hire a non-Jew to do the work and not have a Jew do Melachah on Chol ha'Mo'ed (MISHNAH BERURAH 542:5, SHA'AR HA'TZIYUN 542:8). If the Jewish worker has no money with which to buy food, the employer certainly may hire him to do work on Chol ha'Mo'ed, as mentioned above.
2) HARVESTING ONE'S FIELD DURING CHOL HA'MO'ED
OPINIONS: The Gemara relates that Rav Huna harvested his field during Chol ha'Mo'ed. His son, Rabah, questioned his conduct from the Beraisa which says that harvesting a field during Chol ha'Mo'ed is prohibited (unless one has nothing else to eat). Rav Huna answered that the Beraisa expresses the opinion of Rebbi Yosi, a minority opinion.
The Gemara challenges Rav Huna's assertion that the Tana of the Beraisa is Rebbi Yosi. Rebbi Yosi maintains that a Melachah which is permitted to be done on Chol ha'Mo'ed does not need to be done with a Shinuy, while the Beraisa says that if one must harvest his field on Chol ha'Mo'ed because he has nothing else to eat, he may do so "as long as he does not use cows to thresh." According to Rebbi Yosi, one should be permitted to thresh in the normal manner with cows, since he does not require a Shinuy. The Gemara answers that "since every day we do not use cows to thresh, threshing without cows on Chol ha'Mo'ed is not a Shinuy." Hence, the Beraisa is Rebbi Yosi, and the reason why one may not thresh with cows is not because he requires a Shinuy, but because "even on other days (besides Chol ha'Mo'ed) one does not always thresh with cows."
The Rishonim suggest different ways to understand the Gemara.
(a) RABEINU CHANANEL, RASHI, and most Rishonim explain that there are two different types of Shinuy. The first type is a Shinuy (change) from the normal way in which this act is done. Rebbi Yosi does not require such a Shinuy when one does a permitted Melachah on Chol ha'Mo'ed. The second type of Shinuy is that one must refrain from doing the act in a manner in which people do it when they want to achieve exceptional results. Rebbi Yosi does require this type of Shinuy. Accordingly, when Rebbi Yosi permits Melachah on Chol ha'Mo'ed, he permits it to be done only in a normal manner; he does not permit it to be done in an exceptional manner.
Since threshing with cows (as opposed to with a stick, NIMUKEI YOSEF) achieves outstanding results, one who threshes on Chol ha'Mo'ed must do so with a Shinuy and thresh without cows.
(b) The RAMBAM (Hilchos Yom Tov 7:6, 8:4) gives a different explanation for the type of Shinuy which Rebbi Yosi requires. The Rambam explains that there is a fundamental difference between the allowance to do Melachah for a Davar ha'Aved and the allowance to do Melachah when a person has nothing to eat.
In the case of a Davar ha'Aved, one is permitted to do a Melachah without a Shinuy according to Rebbi Yosi because the Rabanan did not include such Melachah in the category of prohibited Melachah on Chol ha'Mo'ed. In contrast, a Melachah which one needs to do because he has nothing to eat does fall into the category of prohibited Melachah on Chol ha'Mo'ed, but the Rabanan made an exception and permitted this Melachah since the person has nothing to eat. Since it is a Melachah which is prohibited for everyone else, the person for whom it is permitted must do it with a Shinuy. That is why the person who has nothing to eat may harvest only with a Shinuy and may not use cows (see Kesef Mishneh and Lechem Mishneh there). Since the allowance applies exclusively to this person and to no one else, he must use a Shinuy. (See LECHEM MISHNEH, Hilchos Yom Tov 7:6.)
The Rambam apparently had a different Girsa in the Gemara. Perhaps his Girsa was similar to that of the DIKDUKEI SOFRIM. According to his Girsa, when the Gemara explains why cows may not be used even according to Rebbi Yosi, it does not say "since every day we do not use cows to thresh, threshing without cows on Chol ha'Mo'ed is not a Shinuy," but rather, "since all of the people do not use cows for threshing, threshing without cows on Chol ha'Mo'ed is not a Shinuy" (his Girsa omits the word "Ha'idna Nami Lav Shinuy Hu"). Accordingly, the Gemara is saying that since all other people (who do not lack food to eat) may not harvest their fields during Chol ha'Mo'ed, this Melachah remains in the category of prohibited Melachah and a person who has nothing to eat must make a Shinuy when he harvests his field. (According to this Girsa, the word "Ha'idna" is the end of the previous phrase. In addition, the Rambam apparently had the word "l'Ayei" ("it is true") instead of "Lav," so that the phrase reads, "it is indeed necessary to have a Shinuy.") (M. Kornfeld)
3) DOING A PERMISSIBLE MELACHAH IN PRIVATE ("B'TZIN'A")
QUESTION: The Mishnah states that one is permitted to bring his produce into his home on Chol ha'Mo'ed if he fears that it might be stolen if left outside. The Gemara adds, however, that he may do so only in a private, discreet manner ("b'Tzin'a").
Why must he do the act in private? The Mishnah apparently permits this Melachah because of Davar ha'Aved -- to prevent the produce from being stolen. Why does this act of preventing a loss need to be done "b'Tzin'a" while no other Melachah done for Davar ha'Aved needs to be done "b'Tzin'a"?
(a) The RITVA in the name of the RAMBAN explains that in the case of the Mishnah here, the source of the potential loss is external to the object itself (thieves might come and steal the object), in contrast to an ordinary case of Davar ha'Avad where the potential loss is inherent in the object itself (the object will spoil). Consequently, in this case the potential loss is not as evident since no sign of ruin or loss is evident in the object. Since it is not obvious that the Melachah is being done to prevent a loss, and the act can be done easily "b'Tzin'a" without adversely affecting the outcome of the Melachah, one must do the act "b'Tzin'a."
(b) The ME'IRI and the MAGID MISHNEH (7:3) cite the Ramban as well, but they understand his words differently. They explain that in the case of fruit left outside, the potential loss is not inevitable; it is only a Safek Davar ha'Aved. Since the potential loss might not occur at all, one may bring the fruit inside only "b'Tzin'a."
(c) The MORDECHAI (#855) writes that all Melachos done on Chol ha'Mo'ed for a Davar ha'Aved must be done "b'Tzina." (He suggests that a Melachah necessary for the sake of the festival ("l'Tzorech ha'Mo'ed") may be done in public.)
HALACHAH: The REMA in DARCHEI MOSHE (OC 638) cites the Mordechai, but does not mention it in his glosses to the Shulchan Aruch. The SHULCHAN ARUCH (OC 638:2) mentions the requirement of "b'Tzin'a" only for the act of bringing in one's produce (not like the Mordechai).