Generated by SassMeister.com, the Sass playground.

SassMeister-input.scss

// two functions to deal with removing or setting units; 
// aliased to strip() and assert() respectively

@function remove-unit($n) { @return $n / ($n * 0 + 1); }
@function strip($n) { @return remove-unit($n); }

@function set-unit($n, $u) { @return remove-unit($n) * 1#{$u}; }
@function assert($n, $u) { @return set-unit($n, $u); }

.test {
    color: remove-unit(15%);
    width: set-unit(100,px);
    height: assert(200px,em);
    margin: strip(100px) auto;
    }

SassMeister-output.css

.test {
  color: 15;
  width: 100px;
  height: 200em;
  margin: 100 auto;
}