How can I return a Microsoft License delivered with a Dell computer?

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.

You are best to contact Dell as they may offere a Linux option not listed on a site. An OEM license is a Dell product and you would need to deal with Dell. Terms may be differenet depending on country of sale. Normally the COA is affixed to the system so removing to return may not be psossible. Selling an OEM copy to a third party may violate the license agreement as well

The link you provide simply goes to a Dell sales site, possibly one you used to build a config and it has expired.

(I Am Not A Lawyer.)

OEM licences should not be resold. According to Microsoft's EULA (pulled out of the Win7 Home Premium OEM, section 2):

One Copy per Computer. The software license is permanently assigned to the computer with which the software is distributed. That computer is the “licensed computer.”

Licensed Computer. You may use the software on up to two processors on the licensed computer at one time. Unless otherwise provided in these license terms, you may not use the software on any other computer.

Even if you do resell it, it should only work on a Dell computer. Any support should be provided by Dell, not Microsoft, including returns. It would be best, if you really don't want it, to contact Dell before purchase and maybe negotiate a lowered price for the PC to come without an OS.