The half-shekel unites the soul's two portions

בענין מצות השקלים קודם ניסן דכתיב כי תשא כו' ראש בני ישראל כו'.

Concerning the mitzvah of the shekalim before Nisan, as it is written, "When you lift up (ki sisa)... the head of Bnei Yisrael," etc.

The Sefas Emes addresses why the half-shekel is given before Nisan, noting the Torah's phrase "lift up the head" of Bnei Yisrael.

דבר"ה כל באי עולם עוברין לפניו.

For on Rosh Hashanah all who come into the world pass before Him [in judgment].

Rosh Hashanah in Tishrei is the universal day of judgment for all mankind.

אבל ניסן הוא ר"ה למלכים וכל ישראל בני מלכים הם.

But Nisan is the Rosh Hashanah for kings, and all of Yisrael are children of kings.

Nisan is the "new year for kings," and since Bnei Yisrael are bnei melachim, it is their special new year, distinct from the general judgment of Tishrei.

ולכן שולט עליהם קנאת האומות. וצריכין כופר נפש.

And therefore the envy of the nations holds sway over them, and they require a "ransom for the soul" (kofer nefesh).

Their royal elevation in Nisan arouses the nations' jealousy, so Bnei Yisrael need a kofer nefesh — the half-shekel — to protect them.

ואין זה סגולת הצדקה בלבד. רק שע"י השקלים נתרומם ראשן של ישראל ומתעלין לשורש העליון ושם נעשין אחדות אחד.

And this is not merely the virtue of tzedakah; rather, through the shekalim the head of Yisrael is lifted up, and they ascend to the supernal root, and there they become a single unity.

The half-shekel is more than charity: it "lifts the head" of Bnei Yisrael, raising them to their root above, where they fuse into one indivisible unity.

וכ' מחצית השקל ואמרו חז"ל מטבע של אש הראה לו ואמר כזה יתנו.

And it is written "half a shekel," and Chazal said that He showed him [Moshe] a coin of fire and said, "like this they shall give."

The Torah specifies a half-shekel, and the Midrash relates that Hashem showed Moshe a fiery coin as the model — pointing to its inner, spiritual nature.

היינו שיש לבני ישראל חלק בשמים וחלק הניתן להם בעוה"ז.

That is, Bnei Yisrael have a portion in Heaven and a portion given to them in this world.

The "half" hints that each Jew has two halves: a portion in Heaven and a portion here in this world.

וצריכין להתדמות ולהתדבק בחלק העליון.

And they must liken themselves to, and cleave to, the supernal portion.

The avodah is to attach one's earthly "half" to the heavenly "half" through dveikus, making the two into a whole.

וזה הסוד שרמזו חז"ל בפסוק שררך אגן הסהר כמ"ש וישם באגנות וחצי הדם זרק על המזבח.

And this is the secret that Chazal hinted at in the pasuk "your navel is like a round basin (agan ha-sahar)," as it says, "and he placed [half the blood] in basins, and half the blood he threw upon the altar."

The Sefas Emes links the "half-shekel" to the bris at Sinai, where Moshe divided the blood — half in basins, half on the Mizbe'ach — symbolizing the union of the earthly and heavenly halves.

ואמרו שסנהדרין היו יושבין כחצי גורן עגולה.

And they said that the Sanhedrin would sit like a half of a round threshing-floor.

The Sanhedrin's semicircular seating likewise reflects the motif of a "half" that points toward and completes a greater whole.

ובנ"י צריכין להראות זאת כי כל מה שהם שולטין על האומות. הכל כדי שיהיו מכוונים לשורש העליון.

And Bnei Yisrael must demonstrate this — that all of their dominion over the nations is entirely in order that they be directed toward the supernal root.

Bnei Yisrael's elevation over the nations is not for power's sake; its whole purpose is to orient them toward, and connect them with, their root above.

ושיהי' על ידיהם התדבקות התחתונים לעליונים.

And that through them there should be the attachment of the lower beings to the upper beings.

Bnei Yisrael serve as the conduit binding the lower world to the upper world.

ואם באמת כוונתם בעבור הביטול אליו ית' לא יוכל לשלוט ע"ז שום עין רע וזהו כופר נפשו לה':

And if their intention is truly for the sake of bittul to Him, no ayin hara (evil eye) can have any power over this — and this is "the ransom of his soul to Hashem."

When Bnei Yisrael's dominion is genuinely aimed at bittul, surrendering to Hashem, they become immune to the evil eye and the nations' envy. That self-nullifying intent is the true "kofer nefesh to Hashem."

Summary: The half-shekel is given before Nisan because Nisan is the Rosh Hashanah for kings, and Bnei Yisrael — children of kings — are elevated then, arousing the nations' envy and requiring a kofer nefesh. More than tzedakah, the shekalim "lift the head" of Bnei Yisrael, raising them to their supernal root where they become one unity. The "half" shekel (shown as a coin of fire) signifies that each Jew has a heavenly portion and an earthly portion that must be joined through dveikus — echoed by the divided blood of the Sinai covenant and the semicircular Sanhedrin. All of Bnei Yisrael's dominion over the nations exists only to direct them to their root, and when their intent is true bittul to Hashem, no evil eye can touch them — this is the genuine "ransom of the soul to Hashem."