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;
}