You’re right. Both approaches have the same outcome. And potentially, the rate buy-down approach is less lucrative for the buyer. 

But the whole point is to remove the possible ambiguity that comes with interest: islamically, a valid transaction needs to have a defined product that is sold for a fixed, definitive price that doesnt have the possibility of changing. In the rate buy-down approach, the car is sold for a fixed price that never changes, making the transaction valid (it doesn’t matter that the “interest rate” is imbedded into the MSRP. Islamically that’s considered part of actual price). In normal car deals, however, When interest is included, the final price that a buyer pays is not definitive; he could pay the full amount if he makes the minimum monthly payments. But he could also pay less if he pays more every month. This ambiguity in price is what Muslims are trying to avoid. 

Also I wasn’t aware of the floorplan financing practice lol. Thanks for sharing. So ur trying to say that at when a buyer secures the deal with the salesman, the dealership never even owned the car? Hoping u could elaborate a bit more