Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.0.rc.1)
// ----


$map: (
  hello: world,
  goodbye: pittsburgh,
);

@function replace-pair(
  $map,
  $needle,
  $pair
) {
  $new: ();

  @each $key, $value in $map {
    @if $key == $needle {
      $new: map-merge($new, $pair);
    } @else {
      $new: map-merge($new, ($key: $value));
    }
  }
  
  @return $new;
}

@mixin output-map($map) {
  @each $key, $value in $map {
    #{$key}: $value;
  }
}

$test: replace-pair($map, goodbye, (new: pair));

.test {
  @include output-map($test);
}

SassMeister-output.css

.test {
  hello: world;
  new: pair;
}