I hate to burst your bubble, but something isn't right. D plan is below dealer cost even taking the holdback into account. The only way the dealer makes money on A/Z/D plan purchases is because Ford subsidizes them. Without the Ford subsidy the dealer would be losing money for no reason. If they're giving you D plan price then they're making it up somewhere else on rebates, trade-in, doc fees or other fees.
You need to check that paperwork carefully. I suggest getting a "drive-out" price including all taxes, fees, etc. and compare it to an actual X plan price just to be safe.
Not saying it can't happen but in this case the odds are pretty slim.