If you are negotiating with the Dealer, its too late. You will end up buying both sets, even if you go home with one set, whether you think you did or not. The factory is the only place you will come out paying only the difference.
In addition, if its a factory option, your GVWR and cargo capacity rating in the sticker will be adjusted accordingly, and your tire sticker will be correct for the tire you have.
If it just isnt available with that option, I would buy after delivery and just sell the original ones locally.
