the thing is, most of the iphone's have dust behind the screen
every iphone I have seen has 1-3 small specks of dust, i have never seen a perfect iphone
My first iphone had bad light leakage, dead pixels and no dust.
my new one is perfect but there are two pieces of dust, thankfully they are not very centered so they are not annoying
I think the best thing you can do is try to live with it.. you may have to go through a lot of trouble just to get a replacement phone that is just as bad