I'm not sure if this is one of the polymer specimens (essay notes). A good number of different varieties of these have surfaced, mostly birds $20s and a few $50s as well. These have sold for up to $5k so whoever got it for 900 USD got a pretty good deal, with one caveat:
Since, as far as I know, these were never released through official channels, they would technically still be the property of the BoC. Collecting this kind of "leaked" material is in a legal grey zone, for now. I'm not aware of any being confiscated or anyone being prosecuted, but reputable dealers and auction houses will most likely not want to touch them.