Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="someclass notip">notip</div>
<div class="someclass">not notip</div>

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

$dropdown-width: 100px;
$comp-tip: true;

@mixin tip($pos:right) {
  
}

@mixin dropdown-pos($pos:right) {
  &:not(.notip) {
    @if $comp-tip == true{
      @if $pos == right {
        top:$dropdown-width * -0.6;
        background-color: #f00;
        @include tip($pos:$pos);
      }
    }
  }
  &.notip {
    @if $pos == right {
      top: 0;
      left:$dropdown-width * 0.8;
      background-color: #00f;
    }
  }
}

.someclass { @include dropdown-pos(); }

SassMeister-output.css

.someclass:not(.notip) {
  top: -60px;
  background-color: #f00;
}
.someclass.notip {
  top: 0;
  left: 80px;
  background-color: #00f;
}

SassMeister-rendered.html

<div class="someclass notip">notip</div>
<div class="someclass">not notip</div>