The root cause of dust under the screen is because there is gap somewhere in your iPhone case, mainly it happens between the glass and the chrome bezel, and also shows a light leak (I'm not sure on the term they use to describe it).
In a very dark room turn on your iPhone screen preferable with a white image and you will see some light coming out your iPhone and not from the viewable screen.
Like I always say: you are the customer and you got a faulty product, ask for a replacement is your right. Be a customer, don't be aggressive, simply be courteous make your point and remember that you are the customer, ask for the manager in case you need to.