On the first of Adar they make announcement concerning the shekels, for the verse says, "This is the burnt-offering of each month in its month" (Numbers 28:14). The Torah said: renew it and bring Me an offering from a new contribution. And since in Nisan we require the offering to be brought from a new contribution, we anticipate and make the announcement on the first of Adar, so that the shekels may come to the Temple in time. Rabbi Shimon ben Lakish said: It was revealed and known before the Holy One, blessed be He, that the wicked Haman was destined to weigh out shekels against Israel; therefore He set their shekels before his shekels.
Israel's Shekels That Forestalled the Shekels of Haman
Yalkut Shimoni on Torah 386:11
בְּאֶחָד בַּאֲדָר מַשְׁמִיעִין עַל הַשְּׁקָלִים, דַּאֲמַר קְרָא "זֹאת עוֹלַת חֹדֶשׁ בְּחָדְשׁוֹ" אָמְרָה תּוֹרָה חַדֵּשׁ וְהָבֵא לִי קָרְבָּן מִתְּרוּמָה חֲדָשָׁה, וְכֵיוָן דִּבְנִיסָן בָּעִינָן אַקְרוּבִי מִתְּרוּמָה חֲדָשָׁה קַדְמִינָן וְקָרִינָן בְּאֶחָד בַּאֲדָר כִּי הֵיכִי דְנֵיתֵי שְׁקָלִים לַמִּקְדָּשׁ. אָמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ, גָּלוּי וְיָדוּעַ לִפְנֵי הַקָּדוֹשׁ בָּרוּךְ הוּא שֶׁעָתִיד הָמָן הָרָשָׁע לִשְׁקֹל שְׁקָלִים עַל יִשְׂרָאֵל, לְפִיכָךְ הִקְדִּים שִׁקְלֵיהֶם לִשְׁקָלָיו.