Rabbi Pinchas ben Levi said: Because they sold the birthright of Rachel [Joseph], and a tav-coin fell to the share of each one of them, therefore let each one's shekel be a tav-coin. "And they shall give every man a ransom for his soul." "Twenty gerah is the shekel" (Exodus 30:13). Rava said: The biblical sela is three and a third [dinars], for it is written "twenty gerah is the shekel," and we translate this as "twenty ma'in," and it is taught that six ma'ah of silver make a dinar. They raised an objection: But is not the sela of the Sanctuary forty-eight pundyon? What is the nature of this pundyon? It is the surcharge added for the breaking of small coin. This was after they added to it, as it is taught: "twenty gerah is the shekel"—we have learned that the shekel is twenty gerah; and from where do we learn that if one wishes to add, he may add? Scripture teaches, saying "it shall be." One might think that if he wishes to reduce, he may reduce; Scripture teaches, "it [is]" [a fixed amount].
The Half-Shekel as Ransom for the Sale of Rachel's Firstborn
Yalkut Shimoni on Torah 386:8
רַבִּי פִּנְחָס בֶּן לֵוִי אָמַר, לְפִי שֶׁמָּכְרוּ בְּכוֹרָהּ שֶׁל רָחֵל וְנָפַל לְכָל אֶחָד וְאֶחָד טֶבַע, יְהֵא כָּל אֶחָד וְאֶחָד שִׁקְלוֹ טֶבַע. וְנָתְנוּ אִישׁ כֹּפֶר נַפְשׁוֹ. עֶשְׂרִים גֵּרָה הַשֶּׁקֶל. אָמַר רָבָא, סִילְעָא דְאוֹרַיְיתָּא תְּלָתָא וְתִילְתָא הֲוֵי, דִּכְתִיב עֶשְׂרִים גֵּרָה הַשֶּׁקֶל וּמְתַרְגְּמִינַן עֶשְׂרִין מָעִין, וְתַנְיָא שֵׁשׁ מָעָה כֶּסֶף דִּינָר. מֵיתִיבִי, וַהֲלֹא סֶלַע שֶׁל הֶקְדֵּשׁ אַרְבָּעִים וּשְׁמוֹנָה פּוּנְדְּיוֹנִין, פּוּנְדְּיוּן זֶה מַה טִּיבוֹ, קַלְבּוּן לִפְרוֹטְרוֹט. בָּתַר דְּאוֹסִיפוּ עֲלֵיהֶן, דְּתַנְיָא, עֶשְׂרִים גֵּרָה הַשֶּׁקֶל, לָמַדְנוּ לְשֶׁקֶל שֶׁהוּא עֶשְׂרִים גֵּרָה, וּמִנַּיִן שֶׁאִם רָצָה לְהוֹסִיף יוֹסִיף, תַּלְמוּד לוֹמַר יִהְיֶה, יָכוֹל שֶׁאִם רָצָה לִפְחוֹת יִפְחֹת, תַּלְמוּד לוֹמַר "הוּא".