“And Aaron shall present the goat on which the lot fell for Yahweh, and offer him for a purification for sin offering.”
This is a summary description of what is to happen to the two he-goats, preparing for the detail which would follow. It is a favourite device in the Pentateuch for preparing the listener for what is coming and implanting the idea in the mind. Then as the reading of the narrative goes on the hearer is prepared for the important points coming. The goat selected for Yahweh is now offered for a purification for sin offering, but note that the other he-goat is seen as part of that offering (Leviticus 16:5). The two must be seen as part of the one offering, and the way they are dealt with connected together in one picture. (If the High Priest had been able to take one he-goat and divide it in two while keeping half alive, that is what he would have been called on to do).