Page "Surplus value" Paragraph 56

Until payment from sales is received, it is uncertain how much of the surplus-value produced will actually be realised as profit from sales.

So, the magnitude of profit realised in the form of money and the magnitude of surplus-value produced in the form of products may differ greatly, depending on what happens to market prices and the vagaries of supply and demand fluctuations.

This insight forms the basis of Marx's theory of market value, prices of production and the tendency of the rate of profit of different enterprises to be levelled out by competition.