Hasan, thanks for your insightful comments. I largely agree with you. Yes, as you said, "given the development of capitalism, commodity money can be (actually it is!) only burden to capitalist development", when, I want to add, money has emerged as commodity money in its origin. And yes, "fiat system is the most developed and sophisticated system which is in line with the thrust of capitalism for expansion", fiat system which is vulnerable to fluctuation, instability, and even collapse due to the huge departure it, as money, made from its origin i.e. commodity base. I think these observations constitute one of the fatal contradictions inherent in capitalism and specifically in its monetary system. The question I wanted to raise was not whether or not money is commodity only; we know this is not true from our everyday life. Rather my underlying question was whether this, i.e. non-commodity money system which we see everywhere can be sustained indefinitely. I think the apparent inconsistency between Marx's derivation of money as commodity in chapter one and the contemporary capitalism with non-commodity money does not undermine Marx's theory of money, but rather points to the instable and collapse-prone nature of capitalist dynamic.
(will be back with 'commodity as a starting point' soon....)