This sounds like a blending of two separate items:
Ford did in fact buy back some 6.0L vehicles that had excessive warranty claims
Ford offers many buyer loyalty programs that provide a substantial rebate
I don't think there is a specific 6.0L buy back rebate though. A Ford dealer would negotiate a trade-in value for a new/used vehicle sale and other rebates would then also be subject to inclusion during the negotiation for final sales price.
I could be wrong...it's happened before...once or twice...a day sometimes.