Aptronix - Phone dead

I took my ** months old IPhone * to Aptronix store at gachibowli to get the battery replaced, the first time i went it was **.**am in the morning and the staff asked me why do you want to spent **** rs here, bring it to my local store and i will get it repaired for lesser price, he gave me his number also. On *th Augist i went again to the same store and this time another service person denied replacing battery because ringer switch was little loose, later he agreed to replace the battery by asking me to give in writing that if phone does not power on they will not be responsible, i work for a technology company and deal with hardware components on daily basis, battery replacement will never make your device go dead, hence i agreed to it, he took my phone for inspection and told **% it will be fine, So i agreed to pay *krs for battery replacement. On **th Aug i went and submitted my phone on **th Aug when i went to collect it, Guess what it was not powering on, BANG ON, they did it.. and later when i asked to give my old battery and new battery along with my phone they declined and told they will try to fix and took my phone, service people told me that they will call me about the status, but no once called. today on **th Aug i went to the service center again and they handed over a dead phone stating nothing can be done, and declined to give me my old battery. They told that the price they charged was for replacement battery and they will not return the old battery. No where in any documents which was given to me states that they will not return the old part. They are asking me to pay more *K rs to return my old battery.