Lamed Tishrei, 5775
According to a well-known halachic principle, a mitzvas asei takes precedence over a mitzvas lo sa'asei. This rule raises a question with regard to the mitzvah to eat a kazayis in the sukkah on the first night of Sukkos, just like one must eat a kazayis of matzah on the first night of Pesach (Sukkah 27a). What is the halachah if the only bread one has is chadash, i.e., it has been baked from wheat that was grown after Pesach? Does the mitzvas asei to eat in the sukkah take precedence over the mitzvas lo sa'asei of eating chadash?
Tosafos (Kiddushin 38a d"h akruv) quotes a Yerushalmi which writes that there is an exception to the above-mentioned rule: a mitzvas asei that was given before matan Torah is viewed as belonging to a lower category, and as such, it cannot override a mitzvas lo sa'asei that was given after matan Torah. For this reason, continues the Yerushalmi, when the Jewish nation entered Eretz Yisrael in the days of Yehoshua, they were not allowed to eat matzah from the new grain,because the mitzvah to eat matzah on the first night of Pesach was given before matan Torah (Shemos 12:18) while the issur of chadash was relayed afterward (Vayikra 23:14).
One can deduce from this Yerushalmi that one may not eat chadash in the sukkah either.Being that the obligation to eat a kazayis in the sukkah on the first night of Sukkos is derived from the obligation to eat matzah on the first night of Pesach, it has the status of a pre-matan Torah commandment as well and does not override the prohibition of chadash.
However, one can still ask: What is the halachah if one only has a half-kazayis of yashan bread and a half-kazayis of chadash bread? Although the issur to consume less than a kazayis of a prohibited food (chatzi shiur) is min hatorah as well, it is not as severe as consuming a complete kazayis. Can the pre-matan Torah commandment to eat in the sukkah override such a prohibition?
This question can be resolved based on the well-known halachah that one is exempt from sitting in the sukkah if he is mitzta'er (i.e., if doing sowill cause him discomfort).The reason for this is because dwelling in the sukkah on Sukkos is compared to dwelling in one's house the rest of the year. Just like someone would leave his house if it was causing him discomfort, so too, he may leave the sukkah in such a circumstance (see Shulchan Aruch Admor Hazaken §640:5).
However, according to some poskim, the first night of Sukkos is an exemption to the rule, and one must eat in the Sukkah even if it causes him discomfort. Others differ and maintain that he is exempt the first night as well (see ibid. §639:17-19).
Keeping this in mind, let's examine the case of the chadash bread. Now, it is obvious that if one were to sit down to a meal in his home and discover that the only food available was treif, he would rather fast than transgress an issur! Consequently, according to those poskim who rule that a mitzta'er is exempt from dwelling in the Sukkah even on the first night of Sukkos, he should fast rather than consume forbidden bread in the Sukkah.
However, according to those who obligate a mitzta'er to eat in the sukkah on the first night, he would be required to eat the half-kazayis of forbidden bread if no other bread is available.
(Shu"t His'orerus Teshuvah §1:44)