Regarding the claim that Muslims were forbidden from trading... if that's true, then they weren't adhering to the principles of their religion very well!
These levies are called "zakat," a voluntary donation to the Muslim community... well, "voluntary" should also be in quotes... So, this is the same as the tithe in Christianity. The fact that a state cannot exist without taxes is a historical fact. Therefore, taxes were collected in addition to this zakat.
P.S. – Sorry, AlexeyPunk, maybe I'm missing something, but the information you're providing is
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