What did the Chatas atone for?
What is the significance of a goat?
Rashi (22, citing R. Moshe ha'Darshen): To atone for the sale of Yosef, at which time they slaughtered a goat.
Ramban (2, from Bamidbar Rabah 13:14): To atone for Yehudah bringing Yosef's shirt [dipped in a goat's blood] to Yakov.
On what grounds was he permitted to bring a Chatas Nedavah?
Ramban: It was a temporary Heter to permit them to inaugurate the Mizbe'ach with all the types of Korban that are brought on it.