iMac 2017 5K Retina screen smudge

I have a Retina 5k iMac 27 "2017, which I bought in November 2017. These days I noticed in the lower right corner of the screen and specifically on a white background, something like a smudge. I tried to clean it with a cloth, but it seems to be inside. I took a screenshot and saw that it does not appear there. Because I am out of warranty, I was told by apple that this is a common problem and that the screen needs to be changed. I will be charged 880 euros for changing the screen. I read that this may be due to dust or smoke, but the room where the computer is located is always clean and I do not smoke.