It's kind of sneaky, but it makes some sense. It's a rebate, which is essentially them paying you, after the fact. If you look at your paper work they count that as part of what you're putting down, not taken off the price.
If you bought an item for $500 that had a $100 mail in rebate, you're going to pay tax on $500, and get the rebate later, and that's really all that's going on here, except instantly.
It would be nice if they were a bit more upfront about what the repercussions calling it a rebate are, as it wasn't obvious to us until after the fact.
(Although we get to do the paper work again because the dealership sold our car twice. Hurray!)
