I have not read the applicable law, but happen to be meeting with my accountant on Thursday, so i will bring back his opinion after that meeting.
In the interim:
A tax credit generally means that you receive a deduction off your taxes (federal in this case, unless you also have a state program). In other words, you do not receive the cash up front. What you get is a reduction in the taxes that you owe for that year. You either get that in a refund or in a reduced payment, depending upon how much in taxes you owe. In the worst case, you would buy something on January 1st of one year and not be able to get the credit until the subsequent year's filing.
Note that if you owe less than $7,500 in federal taxes, you don't gain anything from a tax credit because you did not owe the government anything in the first place. This is different than a tax rebate or a grant, in which you typically receive a check at, or shortly after, purchase.
Tax credits are paid to the owner of the purchased asset. For this reason a Nissan dealer cannot give you the $7,500. If you buy the asset, you are the one who must clam it. This is different than the lease option. In this case the Nissan Credit company owns your car and can give you the value for the tax credit in the reduced leasing payments because as the owner of the purchased asset, they can claim the $7,500 credit. Since they would usually have a pretty substantial tax payment, they benefit.
In short, if you buy the car, you are likely to have to finance it at the actual pre-tax credit price. You can then claim the tax credit, get a refund (hopefully), and pay that refund into the car loan OR re-finance the loan at that time.
Note: I will try to confirm this on Thursday.
Steve in PA
Reserved 4/20/2010, Forgotten in Pennsylvania