Samsung india — supplied defected product

I bought samsung on nxt from samsung official website shop.samsung.com on sunday 15 jan 2017 and got delivered on 19 jan 2017.my order id is 300001408 when i opened the package and swich on the phone, i noticed that the back button was not working. I called the samsung estore helpline number and told him about the defected phone the executive told me to go to samsung service center and to get a DO letter from there. I on the next day on 20 jan 2017 gone to kotla samsung service center (maa communication) there, the person sitting on the counter checked the phone till that time condition of the phone was proper then he told me that the engineer will check the phone and will make do letter but when he take the phone to engineer and come outside he told that the DO letter of the phone he will not make as there is some scratch in paint at top of the phone. Then again i called the customer care then he told me to submit the phone to service center and get the jobsheet of the phone.then again i gone to service center and submitted the phone.my jobsheet no is 4229584111. now on 25 jan 2017 i got a call from service center that they will not repair the phone untill i pay repairing cost to them.my phone is in full warranty then why should i pay the cost to them.in jobsheet also it is mentioned that the phone is in full warranty. i want that my phone should be replaced because i got a defective phone from samsung company.

regards

shubham tiwari