Sassのif文で変数の代入 ref: http://qiita.com/butchi_y/items/bc7aae615e7bc3ac7900

file0.txt

@each $type in "facebook" "twitter" {
  .#{$type} {
    @if $type == "facebook" {
      $button: "button_like.png";
    } @else if $type == "twitter" {
      $button: "button_tweet.png";
    }
    width: image-width($button);
    height: image-height($button);
    background-image: image-url($button);
    background-repeat: no-repeat;
  }
}

file1.txt

@each $type in "facebook" "twitter" {
  .#{$type} {
    $button: "";
    @if $type == "facebook" {
      $button: "button_like.png";
    } @else if $type == "twitter" {
      $button: "button_tweet.png";
    }
    width: image-width($button);
    height: image-height($button);
    background-image: image-url($button);
    background-repeat: no-repeat;
  }
}