These articles may shed some light on the financial
situation.

New York Times, May 17, 1996
G.M. Buys A Landmark Of Detroit For Its Home http://www.nytimes.com/1996/05/17/us...l?pagewanted=1

USA Today, May 9, 2008
GM buys its Detroit headquarters for $626 million
http://www.usatoday.com/money/autos/...quarters_N.htm