Post: The very first Rashi in parashat Tzav analyzes the word tzav:
|2. Command Aaron and his sons, saying, This is the law of the burnt offering: That is the burnt offering which burns on the altar all night until morning, and the fire of the altar shall burn with it.||ב. צַו אֶת אַהֲרֹן וְאֶת בָּנָיו לֵאמֹר זֹאת תּוֹרַת הָעֹלָה הִוא הָעֹלָה עַל מוֹקְדָה עַל הַמִּזְבֵּחַ כָּל הַלַּיְלָה עַד הַבֹּקֶר וְאֵשׁ הַמִּזְבֵּחַ תּוּקַד בּוֹ:|
|Command Aaron: Heb. צַו. The expression צַו always denotes urging [to promptly and meticulously fulfill a particular commandment] for the present and also for future generations. Rabbi Simeon taught: Scripture especially needs to urge [people to fulfill commandments,] where monetary loss is involved. — [Torath Kohanim 6:1]||צו את אהרן: אין צו אלא לשון זרוז מיד ולדורות. אמר ר' שמעון ביותר צריך הכתוב לזרז במקום שיש בו חסרון כיס:|
Yet, the root of tzav also appears in the word tetzaveh, a word which also has a parasha named after it. And parashat Tetzaveh is earlier, in sefer Shemot:
|20. And you shall command the children of Israel, and they shall take to you pure olive oil, crushed for lighting, to kindle the lamps continually.||כ. וְאַתָּה תְּצַוֶּה אֶת בְּנֵי יִשְׂרָאֵל וְיִקְחוּ אֵלֶיךָ שֶׁמֶן זַיִת זָךְ כָּתִית לַמָּאוֹר לְהַעֲלֹת נֵר תָּמִיד:|
And there, Rashi does not say אין צו אלא לשון זרוז מיד ולדורות. Why the difference? Furthermore, if we wish to step up where Rashi was silent, how shall we explain the tzav in the word tezaveh over there?
This is a question posed by Siftei Chachamim on parashat Tzav:
"Rashi explains in the gemara that it means hurried and zealous. And if you say, why did Rashi not explain this above upon the verse in parashat Tetzaveh? And there is to say that the ziruz [there in Tetzaveh] comes because one needs extra skill to let it shrivel at the top of the olive tree and to crush it with a machteshet so that it should be without dregs."
In terms of why Rashi did not explain this on parashat Tetzaveh, I think this is pretty straightforward. Rashi does not innovate his own midrashim. And here, he is simply channeling Toras Kohanim, otherwise known as the Sifra. The Sifra is only on Sefer Vayikra, and so does not comment on the pasuk in parashat Tetzaveh, which is in sefer Shemot. But Rashi will not innovate, and moving the derasha to another context would be innovating a new derasha from scratch.
It is entirely possible that the midrashic authors cited in Sifra would say something similar on parashat Tetzeveh. And then the zirus could even be about monetary loss, for this involves a donation. Or perhaps not, and tzav in its short form is targeted, rather than the root as found in any form.