Replaced Transmission

Company refuses to honor it's warranty. Their mechanic improperly installed a transmission and they charged to fix his errors. I recently brought in a friend's Nissan Altima to have the transmission replaced and had it charged to my AAMCO Credit Card account. After picking the picking up the auto, the starter made a grinding noise when attempting to start the vehicle. Believing this to be a separate problem I took it to my regular mechanic. He stated that the problem was the screw in the transmission mounting was improperly installed and was causing damage to the starter. We had the car towed to their shop. The manager "Charlie" informed us that since another mechanic looked at the vehicle they would not honor the warranty. This is unacceptable because without having a third party inspect the vehicle how would they be found at fault. It is all around bad business to allow your employees to perform substandard work, and even worse practice not to fix the problem when it does occur.I currently have an open complaint with the better business bureau about the matter.