Samsung - Mobile Phone damaged while given for repair

*** Issue *** :
There was a problem with the bottom panel of my A* mobile ("Home" & "Back" keys) not working. Rest everything was absolutely fine and working. I took it to the Samsung authorized service center in BTM Layout Bangalore (Pristine Electronics & Solutions - contact # *** **** ****). I was told that it would have to be replaced and may cost around INR *K. I was ok with it. Before they opened the phone up, I asked them if there would be any other impact post opening and they answered "NO" apart from fixing few adhesive tapes to be added.
Post opening, they said the part - bottom panel - has moisture (most common and standard reason/excuse by all service centers!) and need to be replaced. However the motherboard that was working fine all along till then had suddenly become non-functional once they touched and opened it! And I was told to pay ~*K for that.
My phone is a relatively new one (just * weeks over a year) and have had no history of any other problem till then.

*** My concern *** :
Service center was non-transparent and honest in telling me upfront that if we open the phone up, the mother-board could also become faulty. If they had told me so, I would not have taken a risk of *-*K to get a *.*-*K problem fixed. I would have lived with the Apps-based Keys to operate my phone.
I asked the Service center to give back the phone to me in the original condition but they refused to do so (basically because of their incompetence and lack of transparency).
One Mr. Mayank Dutt from Samsung Delhi office got in touch with me and after discussions with the Service center, told me that they can NOT get the phone back in the condition that it was in when I given it to them. He offered me **% discount in the charges if I pay the rest. I did not agree to bear the cost of the motherboard (~****+) which stopped working due to service-center's fault.