במדבר, פרק ג׳, פסוק נ״א

The transfer of redemption money from the families of the firstborns to the priests marks the final step in the exchange of the firstborns for the Levites. To determine exactly who would pay the redemption fee and who would be covered by a Levite, Moses needed a system that would prevent arguments. He organized a lottery, placing twenty-two thousand slips of paper reading "Son of Levi" into an urn, along with two hundred and seventy-three slips reading "Five shekels." Each firstborn drew a slip, and only those who pulled the slip demanding payment were required to pay the fee [שפתי כהן].

Because the collected funds were destined for Moses's brother, Aaron, and his nephews, there was a risk that people might suspect Moses of favoring his own family. Observers might think he was trying to enrich his relatives or that he took personal satisfaction in giving them the wealth. To eliminate any doubt, it is heavily emphasized that Moses acted strictly according to the word and command of God. He collected exactly the required amount and transferred it solely out of absolute obedience to God, completely free from any personal or family motives [אור החיים, שפתי כהן].

When it came time to distribute the funds to Aaron and his sons, Moses pooled all the collected money together and divided it into two equal halves. One half was given directly to Aaron, and the remaining half was shared among his sons. This specific method of distribution was based on a precise divine command rather than an equal split among all the individual priests [העמק דבר, מלבי״ם]. By mixing the funds before dividing them, the five shekels paid by any single firstborn were ultimately shared among several different priests [מלבי״ם].