a stain appeared on the back cover

Good day! My name is Vladislav, and I appeal with the next problem. A

week ago I bought an iPhone 7 plus, and on the second day after the

purchase a stain appeared on the back cover. It does not fade or wipe.

No liquid got on the phone.The service recognized this problem as a

cosmetic defect. Experts said that the oleophobic coating is partially

missing. However the store refused to change the phone. Service

technicians advised me to contact technical support. I really hope for

your help! When I’m bought an Apple’s company phone, i’ll want to use a

quality device, and not to go through service and be nervous that i’ll

have to use a low-quality device. Please, help in my problem! Thank you

in advance! I also attach a photo of the problem and the conclusion of

the service.