Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

@mixin testing($properties) {

  @if(type-of(nth($properties, 1)) == list) {
    @each $property, $value in $properties {
      #{$property}: $value; 
    }
  } @else {
    $property: nth($properties, 1);
    $value: nth($properties, 2);
    #{$property}: $value;  
  }
}

.sample-single {
  $properties: background fire-brick;
  
  @include testing($properties);
}

.sample-multiple {
  $properties: opacity 1, background goldenrod, color grey;
  
  @include testing($properties);
}

SassMeister-output.css

.sample-single {
  background: fire-brick;
}

.sample-multiple {
  opacity: 1;
  background: goldenrod;
  color: gray;
}