Hello and thank you for asking your question. My name isXXXXX will try to help.
Belly button, navel, and the umbilicus are all the same things. It is where the placenta attached to the upper abdomen of the puppy. When the mom bites, or tears, the umbilical cord, a little too much force can be applied. So the musculature around the (navel) belly button tears. The tear is a gap in the abdominal muscles. The gap is called a hernia. So an umbilical hernia is a gap, in the abdominal musculature, where the belly button, or umbilicus, is located.
As long as it is smaller than the size of a dime, I would not be too worried.
The larger the hernia, the greater the chance that the intestines, or other intra-abdominal structures, can leak through the hernia and get stuck. That could cause a problem as the blood supply to the stuck structure would be compromised.
So, that rarely happens if the hernia is small.
It can happen in larger hernias though.
I would call the vets, in your area, and get a few quotes for the price of an umbilical hernia repair. Ask for the price of the repair with a neuter if you are getting a male.
Then, the breeder should take that cost off the price of the puppy.
But if you really like the puppy, and the breeder, I would not hesitate to get the puppy if it has a small umbilical hernia.
I hope this helps.
If you have additional questions, please reply.