In Aghanistan, dollars and Aghanis are used interchangeably. Even your grocery store around the corner accepts US Dollars (its owner won't have a cash register but he probably will have a calculator handy to do the conversion). "Luxury" items (for instance prepaid cards for cell phones) have a price tag in USD.
According to yahoo finance, the exchange rate of the dollars to the Afghanis has remained totally unchanged for the last two years. I suppose this means that the Afghan monetary system is based on US currency.