Sounds a bit unfair to all the saffron farmers in other countries. With half the land in Afghanistan suddenly devoted to saffron production, the value of saffron will plummet. Meanwhile, Afghan saffron farmers will be selling their saffron to the US government for some insane price. If I were a foreign saffron farmer I'd be pretty pissed off. Oh, and the number one saffron producer at the moment is Iran, so you'd probably get a vast industry in smuggling Iranian saffron across the border to be sold in Afghanistan.
Oh, and also the drop in opium production will raise opium prices, and so you'll need to keep raising the Afghanistan saffron buying price in order to keep it more attractive than opium farming.
I foresee all sorts of unforeseen consequences, and those are just the foreseen ones!