@extend in @media

gistfile1.md

Sass

%half { width: 50%; }
%full { width: 100%; }

#foo {
  @extend %full;

  @media screen (min-width: 600px) {
    @extend %half;
  }
}

#bar {
  @extend %full;
}

#baz {
  @extend %half;
}

CSS

#foo, #bar {
  width: 100%;
}

@media screen and (min-width: 600px) {
  #foo {
    width: 50%;
  }
}

#baz {
  width: 50%;
}