Inspired by yydl's question.
I read that in Russia where bread was unpurchasable without "bread ration cards", people would save them throughout Pesach and use them after Pesach. One such person heard (before Pesach) from the Lubavitcher Rebbe's father (who was a Rov of Dnepopetrovsk) that such rations are forbidden so he ripped them all up. (It also says that the Lubavitcher Rebbe's father used to tear up his cards before Pesach).
What is the prohibition behind owning such ration cards (as in, why are they different than money which can also buy one bread)?
Based on the answers there (and my opinion before finding out the above information), there shouldn't be a problem with owning such vouchers.
So why is it forbidden?