Good Dog with Aggression Problem, HELP!

I have two dogs, a 12 month old husky mix and a 10 month old boxer/lab/pitbull mix. They are both very good dogs and listen to my every command, and are good around people when I am with them. When I'm at work I keep them in a dog pen in our backyard that is a pretty good size, fenced in about 1/2 or 1 acre. I have had them both since they were 8 weeks old, they have never been abused or had any problems

My neighbors keep their lab with my dogs in the pen sometimes and they get along fine. But when my neighbor comes to get his dog out of the pen, he tells me that my boxer/lab/pit growls and lunges at him and tries to bite at him through the fence. Whenever I am around my dog never does this, he is very well behaved and sits down, and its not that he has a problem with my neighbor because when he is outside of the dog pen even when im not around he likes my neighbor just fine and lets him pet him and doesnt growl or anything. So i have no idea what to do to fix this problem. Hes even neutered, but could it just be the pitbull behavior in him that cant be fixed? or do i sent him to a trainer without me around?

This is going to be a very difficult problem to fix since it doesn't happen when you are present. I think your dog just doesn't want the neighbor's lab to leave and he knows that the actions the neighbor is taking will cause the lab to go away. Maybe he thinks he is protecting the lab because he doesn't know what is being done to him when he gets out of sight. Maybe he just doesn't want his friend to leave. I don't know which is the case. If this is the only time this happens the obvious solution to the problem is for the neighbor to find some other accomidations for his lab. I'm sure some others will come up with other solutions.