GDR citizens used to be able to purchase goods with foreign currency (e.g. the Deutsche mark) in a special chain of shops called Intershop. Now they have to exchange the money for “Forumchecks” before they can buy anything there.
The government has introduced the rule so that it can quickly siphon off the Western money that its citizens bring into the GDR. Visitors from outside the GDR, however, can still pay with foreign currency as long as they show their passport.
Staatsbank der DDR, Französische Straße, 10117 Berlin