My guess would be that the lens was broken at one time, and when it was repaired the exterior housing of a 28mm f/1.8 was used. If you look at the window for the focus marking, it has a depth of field scale. The 20-35mm does not have this.
If the parts are interchangeable, which it appears that they are, I'd say that there really isn't a problem with keeping the lens if you're happy with it.
This however does affect resale value since the seller should be disclosing that it was repaired. If you got a good price for it I'd say don't worry. If you paid top dollar then I might send an email to the seller asking if he'll accept a return or give a partial refund.