Chabad.org — English translation + Rashi
Torah Temimah points to Toras Kohanim (Sifra), Vayikra Dibbura DeNedavah, on 2:1 (nefesh — even Kohen Gadol).
Sefaria — Sifra / Toras Kohanim, Vayikra Dibbura DeNedavah
English: “And a soul” — to include a Kohen Gadol that he may bring a voluntary minchah.
Ribbuy of the anointed kohen — the word “nefesh” includes the Kohen Gadol for a free-will minchah.
Even the Kohen Gadol may bring a free-will minchah. The Sifra reads the term “nefesh” as an inclusive expression that encompasses the anointed High Priest among those who may offer a voluntary flour offering.