The USSR, as a matter of fact, operated under mostly capitalist conditions, capitalism distinguished by:
* The predominance of wage labour in the economy
* The goal of economic activity as the accumulation of capital
* Private ownership of means of production (this is the "mostly" part - the USSR did not have much private ownership, which sets it apart from other capitalisms, but Marx was careful to point out that even under simple "public ownership", society becomes a "general capitalist")
In short: if there is wage labour, it is most definitely not "Communist". Money in its function as money in the circuit of capital, that is, C-M-C', remains money, not merely a symbolic "voucher". Socialists have proposed the idea of non-exchangable labour vouchers, but this is a far cry from real money which was what the USSR had. The ruble was just as much money as the dollar.