Kabul, April 30 : Afghanistan's central bank Da Afghanistan Bank (DAB) sold $14 million by auction on Sunday to stabilise the exchange rate of the national currency afghani, the bank said in a statement released here on Sunday. The local currency afghani has been tumbling against foreign currencies, especially the US dollar, over the past couple of months. The exchange rate of 1 US dollar increased from last week's 86 afghani to 87.15 afghani on Sunday, Xinhua news agency reported. Afghanistan's central bank has injected millions of US dollars into the country's money-exchange market over the past month to prevent the fall of the afghani against foreign currencies. Afghanistan has received more than 2 billion dollars in cash over the past 18 months as part of the international community's humanitarian aid to prevent the economic collapse of the cash-strapped country. /IANS