Have you tried using a gentle leader? You should try that before you take the dog back, because that could completely eliminate the problem. I have a great dane who used to do the same thing. We got a gentle leader so that she could not physically overpower me, and that helped tremendously. If she started to act up, I would just tighten the leash, sometimes to the point where she couldn't move her head. It only took a couple of times before she realized that she didn't like that and she stopped acting up. She still watches the other dogs and is 'on edge' when another dog is nearby, but she doesn't bark or lunge or freak out anymore. Now if we see another dog all I have to do is tell her 'your fine' or 'relax' and she is fine with them. I don't know if this would work with you but I'd hate to see you get rid of her just because of that when it could easily be managed. I'd also suggest taking her to training classes. When she sees another dog in class and starts acting up, the trainer will be right there to show you how to handle her. Hopefully you find something that works so you can keep her
