Moshe's Torah repairs Aharon's blemish in deed

איתא במד' צו את אהרן שמשה רבינו התפלל עליו על מה שנאמר עד עתה בני אהרן חלקת כבוד לעצים עבור בניהם ע"ש.

It is brought in the Midrash on "Command Aharon," that Moshe Rabbeinu prayed on his behalf concerning what was said until now: "to the sons of Aharon" — "You apportioned honor to the trees for the sake of their fruits"; see there.

The Midrash teaches that until this point the offerings had been addressed to "the sons of Aharon," and Moshe Rabbeinu prayed that Aharon himself be honored — comparing the sons to fruit and Aharon to the tree that earns honor through them.

והענין נראה לבאר כי בפ' ויקרא נאמר מצות הקרבנות ובפרשה זו נשנו כל הקרבנות ובכולם נאמר זאת תורת העולה תורת מנחה תורת חטאת כו'.

And the matter appears to be explained thus: in Parshas Vayikra the mitzvos of the korbanos (offerings) were stated, and in this parsha all the korbanos were repeated, and in all of them it says, "This is the Torah (law) of the olah, the Torah of the minchah, the Torah of the chatas," and so on.

The Sefas Emes notes that Tzav restates the offerings already given in Vayikra, but now each is introduced with the word "Torah" — "this is the Torah of the olah," and so on — pointing to a Torah dimension of the korbanos.

נראה כי בבחי' התורה הי' אהרן גדול מבניו רק בבחי' עשי' הי' בו פגם ע"י מעשה העגל.

It appears that in the aspect of Torah, Aharon was greater than his sons; only in the aspect of action (asiyah) was there a blemish in him, through the incident of the Golden Calf.

On the level of Torah, Aharon surpassed his sons. His only deficiency was in the realm of deed, owing to his involvement in the eigel (Golden Calf).

דאיתא עבירה מכבה מצוה ואין עבירה מכבה תורה ולכן רמזו שמשה רבינו ע"ה שהוא בחי' התורה הוא עמד לו להחזירו למקומו ולתקן הפגם בעשי':

For it is brought that "a sin extinguishes a mitzvah, but a sin does not extinguish Torah." Therefore they hinted that Moshe Rabbeinu, peace be upon him, who is the aspect of Torah, was the one who stood up for him, to restore him to his place and to repair the blemish in action.

Since sin can dim the light of a mitzvah (a deed) but never the light of Torah, it was specifically Moshe — who embodies Torah, the dimension untouched by Aharon's lapse — whose prayer could restore Aharon and mend the blemish that the eigel had left in the realm of action.

Summary: Although the offerings were already commanded in Vayikra, Tzav repeats them with the word "Torah" — "this is the Torah of the olah" — because Aharon's greatness lay in the realm of Torah, where his sin of the Golden Calf left no blemish, since "a sin extinguishes a mitzvah but not Torah." His only deficiency was in the realm of action; therefore it was Moshe Rabbeinu, the embodiment of Torah, whose prayer could restore Aharon to his place and repair the blemish in his deeds.