There are several issues here, can neighbor erect a fence on the property line, do you have to pay for half of the fence, what about the fence not being complete, and the neighbor's dog attacking your dog.
That is a lot for this forum. But, in general your neighbor has a right to build a fence, if that fence is faulty and allows his dog to come onto your property and attack your dog he is likely responsible for your damages, and it is unlikely that you have an obligation to pay for the fence which you did not authorize.
There are likely a lot of additional details that need to be discussed. As such, I recommend that you schedule a free case assessment at robandhen.wpengine.com/locations or by calling 303-688-0944 (24/7). At your appointment we can discuss in more detail the facts, your rights, remedies, risks, the law, timeline, options, how we can help, costs, etc.