You have to have a tax liability after all of your deductions. The final total on your tax form has to have you owing the feds $X. Then you can get a rebate of that amount, up to $7,500.00
I guess I'm still a little foggy on this, so pardon my ignorance. I don't have my w2's in front of my, but I would surmise that I owed something around 25k total in federal tax last year.
Now the way I do my withholdings, I probably paid around 28k, so at the end of the year I got back around 3k.
I'm thinking that this federal tax credit would just come off of the amound that I owe the government, so i in my case if everything stayed exactly the same it would be 25k (owed)-7.5k (credit) = 17.5k (adjusted owed). So if I didn't change my withholding (and I wouldn't need to) I would have 28k (actual paid) - 17.5k (adjusted owed) = 10.5 refunded to me at end of year.
If that's completely wrong let me know.