As China's foreign exchange reserves have swelled to unforeseen and uncomfortable levels in recent years, Beijing's policymakers have taken comfort in the thought that they are at least making a paper profit on managing the money.

To keep the renminbi stable, the People's Bank of China buys nearly all the incoming foreign currency, invests it, and then tries to "sterilise" the monetary impact in China by issuing local currency bills to take the funds out of circulation.