I don't collect Germany unless it relates to Poland but did a google & here's what I've come up with.
The overprint was a private production, here's some info in German, the poster bought his set for 10 € a couple of years ago
http://www.philaseiten.de/cgi-bin/index.pl?ST=1225I've re-edited the pic to make it neater

The top row reads"Germany is not viable without colonies"
The bottom row "Give Germany back her colonies"
Your example is similar to the bottom row but on a different stamp from the era
After WWI a set of propaganda labels was sold showing the 19 territories & colonies Germany lost after the war. The set is readily available on
ebay, check out Germany - Lost Colonies.
Three of these labels pertain to Poland, Danzig/Gdansk, Posen/Poznan, Oberschlesien/Gorny Slask/Upper Silesia. Germany regained them during WWII but lost them again afterwards & they are firmly Polish now

I've no idea what your postcard is worth but certainly worth keeping as a record of those tumultuous times