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In that case, that means that the only workable economic system for Marx is a centrally planned economy (which, from what I know, is not the position of the majority of communists). Otherwise, you're going to have severe information transfer/cooperation issues at the system boundaries. Which is already historically what happened in the USSR and most strict application of central planning. And unless I'm mistaken, they still had money.
As for Marx... It's more that I read a subset of Marx works, found too many issues within the theories themselves, and honestly don't have unlimited time to see if he corrects it in some other works. And despite looking a bit for it in other forms (including discussion with some very left-leaning friends), I never found any answer I found really satisfactory.
And to be honest, I understand why you assume this is a propaganda issue : communist/socialist/anarchist theories are largely misrepresented in common discourse. That being said, don't make the mistake of believing that all critics don't know what they're talking about. Or even that mainstream theories are immune to this type of misrepresentation (because they most certainly are not).