מעשה ברבי ישבב שעמד והחליק את כל נכסיו לעניים. שלח לו ר”ג, “והלא אמרו חומש מנכסיו למצות?” ור”ג לא קודם לאושא היה? ר’ יוסי בר’ בון בשם ר’ לוי: כך היתה הלכה בידם, ושכחוה, ועמדו השנים והסכימו על דעת הראשונים. ללמדך שכל דבר שבית דין נותנין נפשן עליו הוא מתקיים, כמה שנאמר למשה מסיני.
ואתייא כיי דאמר רבי מנא: “כי לא דבר רק הוא מכם” — ואם הוא רק מכם, הוא למה שאין אתם יגיעין בתורה. “כי הוא חייכם” — אימתי הוא חייכם? כשאתם יגיעי’ בו.
רבי תנחומא בשם רב הונא: (שמות לה) “ובצלאל בן אורי בן חור למטה יהודה עשה את כל אשר צוה ה’ את משה”. “אותו משה” אין כתיב כאן אלא “אשר צוה ה’ את משה” — אפי’ דברים שלא שמע מפי רבו, הסכימה דעתו כמה שנאמר למשה מסיני.
ר’ יוחנן בשם ר’ בניי: “כאשר צוה ה’ את משה עבדו” כן צוה משה את יהושע, וכן עשה יהושע. “לא הסיר דבר מכל אשר צוה ה’ את משה” — “אותו משה” אין כתיב כאן, אלא “מכל אשר צוה ה’ את משה” — אפי’ דברים שלא שמע מפי משה הסכימה דעתו כמה שנאמר למשה מסיני.

An event with Rabbi Yeshovav, that he stood and divided all his property amongst the poor. Rabban Gamliel sent for him.  “Didn’t they say [that a most] one fifth of his property [should be spent] for mitzvos?”

But wasn’t Rabban Gamliel before Usha [where they ruled this law about one fifth]?

Rabbi Yosi beRabbi Bun in the name of Rabbi Levi: This was the accepted law in their hands. It was forgotten, and the later ones established and agreed to the intent of the early ones. This comes to teach you that anything a court puts their souls into endures, as though it was said to Moshe from Sinai.

This goes like that which Rabbi Mana said: “For it is not an empty thing from you” — and if it were empty, it would be because you didn’t study the Torah deeply. “For it is your life” — when is it your life? Then you do study Torah deeply.

Rabbi Tanchuma in the name of Rabbi Huna: “And Betzalel ben Uri ben Chur of the tribe of Yehudah did all that Hashem commanded Moshe.” It doesn’t say here “that Moshe commanded him”, just “that Hashem commanded Moshe”. Even things which [Betzalel] did not hear from his rebbe’s [Moshe’s] mouth, his idea agreed [with the rest of the Torah, or perhaps: with the Will of G-d] as though it were said to Moshe from Sinai.

Rabbi Yochanan in the name of Rabbi Benayei: “As Hashem commanded his servant Moshe” so Moshe commanded Yehoshua, and so Yehoshua did. “He did not veer from anything that Hashem commanded Moshe” — it doesn’t say here “that Moshe commanded him”, rather “from all that Hashem commended Moshe”. Even things which he didn’t hear from Moshe is ideas agreed [with the rest of the Torah] as though it were said to Moshe from Sinai.

— Yerushalmi Pei’ah 1:1, 3a
(see also a Yerushami Shevi’is 1:5, 2b for a discussion similar to the first part of the above)

… הזורע את שדהו שני מיני חטים: עשאן גורן א’, נותן פאה אחת. עשאן שתי גרנות, נותן שתי פאות.

מעשה שזרע ר”ש איש המצפה לפני ר”ג ועלו ללשכת הגזית ושאלו אמר נחום הלבלר מקובל אני מר’ מישא שקיבל מאבא שקיבל מן הזוגות שקיבלו מן הנביאים הלכה למשה מסיני בזורע את שדהו שני מיני חטים עשאן גורן אחת נותן פאה אחת עשאן שתי גרנות נותן שתי פאות:

… Someone who plants his field with two breeds of wheat: If he make of them one storage in the silo [and thereby treats them as one crop], he gives one pei’ah [corner left over for the poor, in this case from the combined crop]. If he makes of them two storages [treating each breed as its own crop], he must give two pei’os [one from each breed].

An event where Rav Shimon, a man of Mitzpah, planted in front of Rabban Gamliel [such a crop]. They went up to the Chamber of Hewn [Wood, the meeting room for the Sanhedrin in the Beis haMiqdash], and they asked [what to do]. Nachum the Record-Keeper said, “I received from Rabbi Meisha, who received from his father who received from the Pairs [of sages who led the first generations of tannaim, starting with the end of the Great Assembly and of prophecy] who received from the prophets a law [given] to Moshe from Sinai that someone who plants his field with two breeds of wheat: if he makes of them one storage he gives one pei’ah, if he makes of them two storages he must give two pei’os.

— Mishnah Pei’ah 2:4

It seems to me that there are two different means given for how we could receive a law that is considered “halakhah leMoshe miSinai — a law [given] to Moshe from Sinai”.

1. The mishnah states the obvious meaning: Moshe received the law, and it was faithfully transmitted down the ages.
2. The Yerushalmi on the previous chapter gives another possibility — that someone toiled in Torah to discover a result that was certainly given to Moshe, even though it was not then passed on down the generations.

This second possibility requires more analysis.

אמר רבי אילעאי: שאלתי את רבי יהושע, “באלו עומרים פליגי בית שמאי וב”ה?”

אמר לי, “בתורה הזאת, עומר הסמוך לגפה ולגדיש ולבקר ולכלים ושכחו — בית שמאי אומרים ‘אינו שכחה’; ובית הלל אומרים, ‘שכחה’.”

וכשבאתי אצל רבי אליעזר, אמר לי, “לא נחלקו בית שמאי ובית הלל על העומר שהוא סמוך לגפה ולגדיש ולבקר ולכלים ושכחה, שהוא שכחה. ועל מה נחלקו? על העומר שנטלו ונתנו בצד הגפה, בצד הגדיש, בצד הבקר, בצד הכלים, ושכחו. שבית שמאי אומרים, ‘אינו שכחה’, מפני שזכה בו; ובית הלל אומרים, ‘שכחה ‘.”

וכשבאת, והרציתי את הדברים לפני רבי אליעזר בן עזריה, אמר לי, “הברי’ הן הן הדברים שנאמרו למשה בחורב.”

Rabbi Ilai said: I asked Rabbi Yehoshua about which sheaves the Beis Shammai and Beis Hillel disagree. He said to me, “In this Torah: a sheaf that is next to a wall, a stack, a heard or utensils and [the owner] forgot it. Beis Shammai say, “It is not shikhekhah [and thus not sufficiently forgotten for the owner to obligated to leave the sheaf for the poor].” Beis Hillel say, “It is shikhekhah.”

When I went to Rabbi Eliezer, he said to me, “Beis Shammai and Beis Hillel did not disagree about a sheaf that is next to a wall, a stack, a heard or utensils and [the owner] forgot it, that it is shikhekhah. About what did they disagree? About a sheaf that he picked up and placed on the side of a wall, the side of a stack, the side of a heard or the side of utensils. That Beis Shamai say, “It is not shikhekhah” because he put significance to it. Beis Hillel say, “It is shikhekhah” [because the reminder he used is mobile, and not guaranteed to be there later anyway].

And when I came and presented these ideas before Rabbi Elazer ben Azariah, he said to me, “By the Creator! These are the very things that were said to Moshe in Choreiv.”

— Yerushalmi Pei’ah 6:5, Vilna ed. 29a

Here we have the explanation of the scope of a dispute between Beis Hillel and Beis Shammai as being given to Moshe at Mount Sinai (a/k/a Choreiv), and yet the two schools obviously couldn’t have had that dispute until a millennium after the revelation!

But the gemara doesn’t speak of a “halakhah” given to Moshe, but rather “hadevarim” — echoing the first half of the voice from heaven “אלו ואלו דברי א-לוהים חיים הן, והלכה כבית הלל — These and those are the words of the Living G-d, and the halakhah is like Beis Hillel”.  Perhaps Rabbi Elazar ben Azariah was saying that this understanding of the dispute captures at least part of the plurality of thought that Hashem gave Moshe on the topic of grain forgotten near movable but significant items.

In terms of the two types of leMoshe miSinai, though, this truth — an understanding of a dispute a millenium later — must be of the discovery of a truth sort (type 2), and couldn’t possibly have been handed down teacher to student since Horeb.

The Rambam has a famous difficult statement with respect to halakhos leMoshe miSinai. He writes that they are never touched by machloqes. The obvious difficulty — there are countless counterexamples.  (For example, the list of deformities that would render an animal tereifah and thus make any shechitah irrelevant is both halakhah leMoshe miSinai and the subject of numerous disputes.)

It doesn’t fit in the words of the Rambam, but I wish could have used the above distinction to resolve the question. Within halakhos that we know are miSinai because Moshe told Yehoshua who told the Zeqeinim and to on down the generations could in theory lose their details in transmission, and machloqes could ensue.

However, through true yegi’ah beTorah (as the first Yerushalmi puts it, above) one can rediscover a truth that we know must have been given to Moshe. If that truth is a halakhah (rather than a spectrum of divrei E-lokim Chaim), then we would only realize its miSinai nature because it is so clearcut in hindsight that no one would consider an alternative position.As the Talmud puts it, “ללמדך שכל דבר שבית דין נותנין נפשן עליו הוא מתקיים, כמה שנאמר למשה מסיני — to teach you that any matter that a beis din gives over their souls to it endures, as though it was said to Moshe from Sinai.”

As I wrote, though, this can not be the Rambam’s meaning. His exact words in his introduction to his commentary to the mishnah (pg 11 in the Qafech edition) are “כל זמן שיאמר אדם קבלתי כך וכך — any times that a person says ‘I received such and such…'” It is explicitly a received halakhah leMoshe miSinai, and not one discovered through yegi’ah.

Still, the Rambam’s position is difficult as at face value it contradicts statements he himself makes elsewhere. And most other rishonim dispute it. So perhaps this suggestion stands as a possibility without his great name attached to it.