I found this interesting item at auction and had to get it.
In 1923, the German Postal Ministry conducted a sale of old German postage to help fund its reparations payments to the Allies. This envelope is from that sale.
The envelope originally contained stamps of the
1900 and
1905/15 Kaiser's Yacht series from the German colony in Kamerun.
Sale price was 19,000 Mark. With the hyperinflation of the period, that was around US$1. At the lower right of the envelope is a pencil notation of the date (10 March 1923) and price paid.

The back has the seal of the Reichspostministerium Markenverwertungsstelle, the office of the German Postal Ministry responsible for old stamp sales.