I was suspicious too, but it turns out OP is correct. This is a much better article detailing the offer. The real question, according to the authors was whether the offer was authentic or a trap:
Ein Geheimdokument belegt: Im Sommer 1990 sondierte ein sowjetischer General (Geli Batenin), ob Bonn Interesse an Ostpreußen habe. War das ernst gemeint oder eine Falle?
This is the article that is cited in the book that the OP cites below (i.e. the author of the book cites this different Der Spiegel article from 2010 as the source of his information). Der Spiegel - for those unfamiliar is comparable to Time Magazine - cites the memo as "Fernschreiben Nummer 2585" from the archives of the German Foreign Ministry. I'm in Berlin and have access to the Foreign Ministry archives - if I were sufficiently motivated, I could go look this up.
To directly answer OP's question: Any such deal between the USSR and Germany would have required the consent of the Western Allies (Britain, France, and the United States), as Germany did not have full sovereignty until the so-called 2+4 treaty that officially ended the official occupation of Germany following World War II. Because the Western Allies would have never accepted such a deal, it was a non-starter, and that's assuming it was a legitimate offer in the first place. As for timing, the offer of the Soviet General came in Summer 1990, the 2+4 agreement was signed in September, and Germay regained full sovereignty the next year.
Because there never was such an offer. Since that "offer" was new to me i had to google and the only reference i found was this article from 1999 in the german Spiegel:
http://www.spiegel.de/spiegel/print/d-14763879.html
Therein the then governor of Kalinigrad states that the offer was a rumor circulating in the baltic countries.
[Google translated]
SPIEGEL: The reason for this discussion were rumors, Gorbachev offered the Chancellor Kohl to buy Kaliningrad for 70 billion marks.
Gorbenko: There were many such rumors, spread by politicians in neighboring countries or at the level of small parties. I know of no official statement like that. In Gorbachev's two-plus-Four-Treaty of 1990 it comes to the territory of the Federal Republic, and since the Kaliningrad region does not occur. The contract relates to the inviolability of borders in the postwar years. Since you can just also remember many other things. The history of Europe is the history of Europe's borders.