Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v0.7.0)
// ----

// Sass Styles to style external links.
// $url string interpolated into background-color, but not selector?

$url: 'site.com';

a[href^="http://"]:not([href*="#{$url}"]).cta,
a[href^="https://"]:not([href*="#{$url}"]).cta {
  background-color: #{$url};  // string will echo correctly
}

SassMeister-output.css

a[href^="http://"]:not([href*="#{$url}"]).cta, a[href^="https://"]:not([href*="#{$url}"]).cta {
  background-color: site.com; }