Update:
I realize that the IRS does not care; the dealer insisted it was a Florida thing.
Called the Florida Dept. of Revenue, and they had no idea what I was talking about. They did some digging and could find no protocol for paying tax on a rebate. The tax is paid at time of sale.
I needed to speak to the Ford dealer that sold me my trade in originally; the finance guy there also said he has never heard of that, and he is in a state with a state income tax.
An extensive internet search shows nothing beyond the IRS rules that point out that it is not considered income (Pub. 525, quoted by another poster).
So at this point I can't help but think it was an underhanded way to put more money in their pocket.
I am going to follow up with management at the dealership and try to get them to show me where it says I have to pay taxes on a rebate in Florida (it does get taxed in some other states apparently).
Depending on how that goes will decide the next course of action.
If they are being deceptive, they certainly need to be called out on it, and buyers need to be aware of it. I am not yet naming the dealer in the event it is all legit, but will if it is not.
Thanks everyone for your thoughts. Strange situation!