Is the mitzvah of Oneg Shabbat Torah or rabbinic in origin? Depends on who you ask.

Eight rabbis on Today’s daf TB Yevamot 93 holds the position that one may make a transaction on futures. “Rav Naḥman bar Yitzḥak said: Rav Huna agrees with the opinion of Rav, and Rav agrees with the opinion of Rabbi Yannai, and Rabbi Yannai with the opinion of Rabbi Ḥiyya, and Rabbi Ḥiyya with the opinion of Rabbi Yehuda HaNasi, and Rabbi Yehuda HaNasi with the opinion of Rabbi Meir, and Rabbi Meir with the opinion of Rabbi Eliezer ben Ya’akov, and Rabbi Eliezer ben Ya’akov with the opinion of Rabbi Akiva, who said: A person can transfer to another an entity that has not yet come into the world. This shows that Rabbi Akiva indeed maintains that one can acquire something that does not yet exist.- אָדָם מַקְנֶה דָּבָר שֶׁלֹּא בָּא לָעוֹלָם” (Sefaria.org translation) Only Rav Naḥman bar Yitzḥak holds the position that one cannot acquire something that does not yet exist.

The Gemara then brings the proof for each one of those eight rabbis. Based on Rabbi Yannai’s story Rashi comes to a conclusion that disagrees with Rambam’s understanding of the nature of Oneg Shabbat. First the story:

“Rabbi Yannai also agrees with the opinion of Rabbi Ḥiyya, as demonstrated by the following episode: Rabbi Yannai had a sharecropper working his land who would bring him a basket [kanta] of fruit every Shabbat eve. One day he was late and did not come. Rabbi Yannai took tithe from the fruit in his house for the fruit he expected to receive. He did this in case the fruit arrived near the beginning of Shabbat, as one may not tithe on Shabbat. However, Rabbi Yannai was uncertain whether it is indeed possible to separate tithes for an entity that has not yet reached one’s possession. He therefore came before Rabbi Ḥiyya to inquire whether his separation of tithes was effective.

“Rabbi Ḥiyya said to him: You acted well, as it is taught in a baraita, with regard to a verse that discusses tithes: “And you shall eat before the Lord your God…in order that you should learn to fear the Lord your God all the days” (Deuteronomy 14:23). With regard to the emphasis of “all,” these are Shabbatot and Festivals. With regard to what halakha was this stated? If we say it was stated in regard to the issue of tithing and eating on Shabbat, this halakha is redundant. Was a verse necessary to permit the prohibition against moving objects, which applies by rabbinic law? Since the prohibition against moving objects is from the Sages, the Torah is certainly not referring to this halakha. Rather, is it not referring to a case like this, of one who tithed an entity that was not yet in the world, in honor of Shabbat?” (Sefaria.org translation)

One may not eat on tithed fruit. Rashi ד"ה אֶלָּא לָאו, כִּי הַאי גַוְונָא comments that one is permitted to tithe fruit that is not yet come into existence hey Jason what’s up but will in the future for the purpose of oneg Shabbat, delighting in the Shabbat. Because of all the other cases of the Gemara are Torah law, we have to conclude that here Rashi holds that Oneg Shabbat is deorita (דאורייתא) a Torah law and not rabbinic ordinance (דרבנן).

Rambam disagrees. He writes: “Four things have been enjoined regarding the Sabbath: two on biblical authority, and two on the authority of the sages and clearly expressed by the Prophets. The Torah says: Remember (Exodus 20:8) and Observe (Deuteronomy 5:12); the Prophets clearly speak of Honor and Delight (Oneg), as it is written: "Call the Sabbath a delight, and the Lord's sacred day an honor" (Isaiah 58:13).” (Mishneh Torah, Hilkhot Shabbat, 30:1, Sefaria.org translation)