If the spot is not large you can use a small amount of bleach. Use a damp cloth with the bleach and leave it sit on the spot and dab it it carefully to remove the detergent dark spot. If this does not work you can dab it with a little denatured alcohol and see if it will lighten up other wise you may have to sand the area to remove the spot and touch up with shellac. You may have to sand the the whole top and use a little stain to even out the color and then top coat with shellac assuming the finish is shallac.