It isn't a stamp. It's a "Spendenmark." As I understand the system, you contributed money to the organization, in this case Nationale Front, and received this "sticker" in exchange. There are a number of organizations that issued them. They were collected into albums.
Yours is from a 1949 series, "Vaterland"
http://www.ddr-spendenmarken.de/NF/nf_13.phpEdit: I forgot to add that the "5" indicates that it is a picture of the main train station in Leipzig.
Dan
