On a cropped sensor camera like the D90, I personally find a 35mm focal length more useful, and if I were buying I would go for the 35mm f/1.8 at half the price.
Optically, my friends who have the 50mm f/1.4 say it's a bit soft wide open, but has nice bokeh. According to this review it is quite soft wide open, and sharpest at f/5.6.
On the other hand, the 35mm f/1.8, like the 50mm f/1.8, is very sharp wide open, and at maximum sharpness by about f/2.8 (see this review). I think the bokeh on the 35mm f/1.8 is pretty good, Ken Rockwell disagrees
If you can't pursue the seller and work things out, then the money is effectively lost already - you've overpaid for the 35mm lens, or you'll lose out by selling it and buying a 50mm f/1.4. Just put that behind you and try to forget how much you (over)paid for the lens, and just unemotionally try to work out where to go from here.
My advice would be use this lens for a bit and see how you like it. Will you use the extra stop that the f/1.4 gives you? If you do a lot of low light shooting maybe. If it's soft and you often have to stop it down, then I'm not sure it's such a great advantage. If you need the faster lens, or you prefer 50mm focal length to 35mm focal length, then sell this one and buy the other.