!global

global.css

.test {
  color: green; }

.test-global {
  color: blue; }

global.scss

$foo: red;

.test {
  $foo: green;
  
  color: $foo;
}

.test-global {
  $foo: blue !global;
  
  color: $foo;
}

shell

$ scss global.scss global.css
DEPRECATION WARNING on line 4 of test.scss:
Assigning to global variable "$foo" by default is deprecated.
In future versions of Sass, this will create a new local variable.
If you want to assign to the global variable, use "$foo: green !global" instead.
Note that this will be incompatible with Sass 3.2.