CSS property as SASS mixin value [duplicate]

Answer 1

If you want to use variables as property names you need to use string interpolation - #{$var}.

So this should work:

[class*="shift"] {
  $sft-o: 10px;
  @mixin shift_stp($val) {
    &[class*="_sml"]{ #{$val}: $sft-o; }
    &[class*="_mid"]{ #{$val}: $sft-o * 2; }
    &[class*="_big"]{ #{$val}: $sft-o * 3; }
  }
  &[class*="_m"]{
    @include shift_stp(margin);
  }
  &[class*="_p"]{
    @include shift_stp(padding);
  }
}

DEMO

Just a note: for your attribute selectors ... *="_m" will also apply to the ones that have _mid in them (see here) ... so maybe you should rethink this a little.