pg_dump and -U flag.

I am trying to use pg_dump as user root but using another username for the
database.

I set USER="postgres" and exported it. It still tries to login using the
root username. PGPASSWORD can be set and exported, is there a way I can do
this for the username?

Also, could it be possible for someone to program support in pg_dump to
have a -U flag/argument to set the username that pg_dump will use?

p.s. I'd like not to use -S and expect.

Thankyou.