Kabul, March 13 : Afghanistan's central bank said on Monday that it sold $16 million by auction in a bid to stabilise exchange rate of the national currency afghani. The afghani has been tumbling against foreign currencies, especially the .S dollar over the past several months, reports Xinhua news agency. The exchange rate for $1 increased from last week's 88 afghani to 88.50 afghani on Monday. The Da Afghanistan Bank (DAB) has injected millions of US dollars into the country's money-exchange market over the past month to prevent the fall of afghani against foreign currencies. The Afghan central bank also auctioned $16 million last week. The war-torn Afghanistan has received nearly $2 billion in cash over the past 17 months as part of the international community's humanitarian aid to stabilise its economy. /IANS