“Thus, when the industrial cycle is in the phase of crisis, a general fall in the price of commodities is expressed as a rise in the value of money, and, in the phase of prosperity, a general rise in the price of commodities, as a fall in the value of money. The so-called currency school concludes from this that with high prices too much, with low prices too little money is in circulation. Their ignorance and complete misunderstanding of facts are worthily paralleled by the economists, who interpret the above phenomena of accumulation by saying that there are now too few, now too many wage-labourers.” [Capital, Volume I, Chapter 25]

The most common way in which the value of money is undermined is through the creation of “Fictitious Capital” i.e. money or credit which is created without any commeasurate increase in the number of commodity values in circulation.

We can see this at the moment with the bursting of the speculative bubble on the world financial markets, which although it manifests itself to most people in the form of higher property or oil prices is at the end of the day actually a reflection of the declining value of currency, rather than a change in the value of those commodities (food prices are a little more complicated, because although food prices are artifically high in NZ this is because of the relatively lower labour inputs in agriculture in this country compared to say Europe).

See further:

http://marxists.org/archive/marx/works/1867-c1/ch25.htm
http://marxists.org/archive/marx/works/1859/critique-pol-economy/ch02c.htm