Ok, so I had never heard of this myself and did some looking into it. A couple of recommendations I found were to
1) check the quality of food the dog is on. Just like our skin, dogs skin discharges toxins and what not, so there may be something in the food that's causing the excessive moisture in the armpit and belly area.
2) Have a full thyroid panel run - again, excessively moist skin can be a sign that the thyroid is acting up. Dachshunds are prone to hypothyroidism, so that may be a warning that something is not quite right. If your pup is fixed, the chances of hypothyroidsim go up.
Hope you find out what's bugging your little guy. Keep us posted