Musar zu Wajikra 22:18

This still leaves open the question why the Torah chose the word אדם instead of איש. Granted that, had the Torah spoken about a mandatory עולה offering (which Gentiles are not allowed to bring), I would have thought that the designation אדם was chosen in order to emphasize that only Israelites are included because the title אדם does not apply to Gentiles. We further know from the use by the Torah of the term איש איש (Leviticus 22,18), in connection with קרבן נדבה, that our sages rule that such offerings by Gentiles are acceptable (Menachot 73). Because the Torah in our verse speaks about free-will offerings why would the word איש instead of אדם not have been appropriate?