I believe TECHNICALLY per the terms of the Windows OEM EULA, you're supposed to be able to contact Dell and say 'I don't want this OEM license' and get a refund for the difference. PRACTICALLY, doing that is going to be almost impossible. I've heard of it happening but it takes months of phone calls, etc, it really is an enormous pain. Not only does it require the involvement of divisions that you can't call directly, but they really don't want to fuss with it.
It is a Dell OEM copy, so Microsoft has no interest in it. I would also point out that it's already worked into the price of the machine, and you won't be saving a significant amount of money this way.
You cannot resell an OEM license like this; not only is it against the EULA, but it won't work anyway. The license key on the sticker is really just for show and will not work with an off-the-shelf copy of Windows - when you install with the Dell custom image you should never actually be asked to enter it, and if you enter it on a regular copy of Windows it will not accept it as valid. (Note that this isn't true for system builder OEM packs, just ones from giant bulk volume vendors like Dell).
EDIT: Clarification from the comments: when I say these keys won't be valid, I mean for internet activation. If you want to call you can possibly get the authentication string.