I use
sass --watch scss:css
to have Sass automatically create CSS files (and put them in the /css directory) for each SCSS fi
Try adding @charset "UTF-8"; to your original SCSS file, SASS shouldn't override it/add its own then.
Like @Alireza-Fatahi states in this answer, you could save a line in CSS output and instead go for a default Charset on external files, by adding this line to your config.rb in your project folder:
Encoding.default_external = "UTF-8"