Siman 342

The Rambam wrote in chapter 24, law 10: "All matters that are forbidden due to shevut—they did not decree upon them during bein hashemashot; rather, it is on the day itself that they are forbidden, and not during bein hashemashot, provided that there is a matter of a mitzvah or a case of urgency. How so: it is permitted for him during bein hashemashot to climb a tree or to float upon the water to bring a lulav or a shofar, and likewise he may bring down from a tree or bring out from a karmelit an eruv that he made, and likewise if he was preoccupied and rushed and needed a matter that is a shevut during bein hashemashot—behold, this is permitted, but if there was no urgency and no matter of a mitzvah—it is forbidden. Therefore, one may not tithe definite produce during bein hashemashot even though the prohibition of separating the tithe on Shabbat is due to shevut, but one may tithe demai." Thus far his words.

The explanation of his words: For the sake of a mitzvah, it is always permitted to perform a shevut during bein hashemashot, and even for a voluntary matter, if one was preoccupied and hurried and could not have done it beforehand—it is permitted even for his own needs. For indeed, from the wording of the Shas in several places where it said that they did not decree a shevut during bein hashemashot, it implies that it also refers to a voluntary matter. And so it implies from the wording of Rashi (8a, s.v. "v'Rabbi") who wrote: For the shevut that the sages decreed regarding the prohibitions of Shabbat, they only decreed during the certain Shabbat, etc., see there. However, since we do not find this matter in all places except regarding eruvei techumin, and we hold that one only establishes an eruv for the sake of a mitzvah—and even not for a complete mitzvah but to greet his fellow and even to stroll, as is written in siman 415—and we also hold that if one established an eruv for a voluntary matter, his eruv is a valid eruv, therefore the Rambam, from his great wisdom, decided that for the sake of a mitzvah it is always permitted, and for a voluntary matter if he was hurried and preoccupied. But for a voluntary matter with ease, when he could have done it beforehand—certainly they did not permit it.

This is the wording of the Tur: "Every rabbinic prohibition is permitted during bein hashemashot for the sake of a mitzvah, such as to take his eruv that is placed in a karmelit, or to climb a tree after it, or to take from it a lulav placed upon it, or if he was preoccupied and needed to tithe fruit during bein hashemashot. And so wrote the Ri, that it is permitted to tell a non-Jew to light a candle for him during bein hashemashot." Thus far his words. From his words it is implied that only for a matter of a mitzvah, but for an optional matter, it is not permitted under any circumstances. And he also holds that this which is permitted to separate demai during bein hashemashot is specifically when he was preoccupied, but certainly vadai is forbidden even in such a case, and according to the opinion of the Rambam, even vadai is permitted in such a case. And the Tur holds that to tithe is not called a matter of a mitzvah at all, for he could eat other things on Shabbat. However, it is certainly true that if he has nothing to eat, even the Tur admits that he may tithe during bein hashemashot. And even a yahrtzeit candle is permitted to tell a non-Jew during bein hashemashot to light for him (Taz in the name of the Rashal, and for the explanation of the words of the Tur see Beit Yosef and Bach and Derisha and Taz, and what appears to my humble opinion I have written, and examine it closely).

And know that there is one who is uncertain if even on Saturday night during bein hashemashot we say this, that they did not decree during bein hashemashot, or since the Shabbat has already been established, it is forbidden (Magen Avraham). And from Tosafot in Eruvin (34a, s.v. "ve'amai") it appears that there is no difference, see there (Tosefot Shabbat), and also from the fact that the Shas and the poskim wrote without qualification, it is clear that there is no difference. And so it is explicitly clear from the wording of Rashi and the Rambam that we brought, who wrote: that they only decreed upon the day itself and upon the certain day, and consequently, they also did not decree upon the bein hashemashot of the conclusion of the day.