Negate a numerical variable and add 'px' to it in LessCSS

Just multiply by 1 in the sign and units you want. So:

.sprite-size(@width, @height, @x, @y) {
  width: @width*1px;
  t: @height*1px;
  background: @sprites no-repeat  @x*-1px  @y*-1px;
}

You can also try this:

.sprite-size (@width,@height,@x,@y) {
  width: ~"@{width}px";
  height: ~"@{height}px";
  background: @sprites no-repeat  @x*(-1px)  @y*(-1px);
}