Will it make a difference? I opened the line to get a discount on my purchase, and then found out it was hurting my FICO score as a new account. Will it help my score if I immediately close that account?
The damage is done now with the inquiry and the impact on your overall account age. You won't get any benefits from closing it, FICO-wise, especially if you have a balance. You may as well let it age and contribute to your history and util - unless of course there are some crazy account fees that would prompt you to close it.