Firstborn sons and firstborn donkeys belonged to Yahweh, but Yahweh did not want them sacrificed to Him. Instead, the Israelites were to sacrifice a lamb in their place. This allowed the Israelites to buy the donkeys and sons back from Yahweh.
God speaks of the offering as if the person was to carry it in his hands. AT: "No one may come to me without an offering" or "Everyone who comes to me must bring me an offering" (See: [[rc://en/ta/man/translate/figs-metonymy]])