I would use "a" if an article is needed at all. That would depend on context.
We use "a" but not "an" for unicorn and uniform.
US$ means United States Dollar.
Due to the "united", we should use "a" instead or "an", right?
Why is it, I read a news article stating "an US$80 million" but not "a US$80million"?
"An $80 million dollar..." is correct. The "US" is just denoting that it is American rather than,say, Canadian or Australian dollars.
I am not a teacher.
This is an interesting point.
I naturally read 'an US$80 million...' as 'an eighty million dollar...'.
Couldn't it be assumed that the writer of the article had that in mind too? If he wanted people to read 'a US dollar eighty million...' he would have used 'a'.
I would like to see the entire sentence in which it appeared.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.