Exploitation of workers does not equal capitalism.

Capitalsim is the free exchange of goods and services, with prices and pay determined by a free market.

If you have a manager under socialism secretly stealing from the gov't and/or the employees, that is just corrupt socialism, as there is still no free market.

If a free market, say a capitalist black market (goods, services, and labor), were to arise, there would be a case along the lines of what you are suggesting. But, this, then, would just be a rejection of socialism, replacing it with capitalism.