px_fallback_example.scss

@function apply-fn($fn, $values) {
  $new-values: ();
  @each $v in $values {
    $new-values: append($new-values, call($fn, $v), list-separator($values));
  }
  @return $new-values;
}

@function rem2px($value) {
  @if unit($value) == rem {
    @return $value / 1rem * 16px;
  } @else {
    @return $value;
  }
}
@mixin px-fallback($property, $values) {
  $fallback: apply-fn(rem2px, $values);
  @if $fallback != $values {
    #{$property}: $fallback;
  }
  #{$property}: $values;
}

@mixin margin($value) {
  @include px-fallback(margin, $value);
}

.with-fallback {
  @include margin(2px 0.5rem 4px);
}

.no-fallback {
  @include margin(2px 9em 4px);
}