Samsung Smart Care Centre Old Batteries?

I always buy and support Samsung products because of their reliability. I went to Samsung Smart Care Pietermaritzburg to get my Galaxy S7 Battery changed. I paid the premium price to get good service. However, 1 - 2 months down the line, this battery is worse than the one it replaced! It lasts only 8 hours on a full charge. I'm convinced I was not given a brand new battery. When I asked the consultant to give me my old battery they said they cant because Samsung requires it back, a but strange isnt it?, how do I know I was given a brand new battery? I want a brand new battery put into my phone. Samsung please sort this out.