<===> input.scss
$variable: 1;

$foo: #{$variable}px;
$bar: #{1}px;
$baz: "1px";

div {
  top: -$foo;
  top: -$bar;
  top: -$baz;
}

<===> output.css
div {
  top: -1px;
  top: -1px;
  top: -"1px";
}