The White bumper is a one peice A3 bumper
The red one is a two peice, it has a seperate valance. you can see the join just above the diffuser. It is a 2004 non s-line upper bumper, with a 2007 S3 valance and 2007 S3 diffuser. You can see that the two parts dont really fit properly.
It will not fit your white bumper im afraid.
My advice would be to return that white bumper or sell it on.
Buy a 2010/2011 S3 Bumper, Valence and dissuser (3 parts). It is a much better looking diffuser than the 2007 one. and will 100% fit your car
Used body parts... Unless it is the correct colour, dont bother. if you need to respray, its easier to buy brand new parts. And they wont have been damaged by couriers.
Part numbers... If you buy a used ebay bumper, make sure you can see a photo of the actual part number molded into the bumper. they are frequently listed wrong. ( alot of hassel)
Hope this helps