Aliya Summary: G‑d then tells Moshe to count all the firstborn Israelites — because the holiness of each Israelite firstborn was now to be “transferred” to a Levite. The census revealed that there were 273 more firstborn than Levites. Each of these “extra” firstborns (as determined by a lottery) gave five shekel to the priests, and was thus “redeemed.”
Rashi points out that the 5 shekalim of the firstborn, each was worth 20 pieces of silver, the amount the brothers received for the sale of Yosef. Rashi considers there to be a connection between the sale of Yosef and the requirement of redemption of the firstborn. Remember that Yosef was Rachel’s firstborn.