laxmi sathyanarayana
Apart from what already been suggest, you can use different sales area to differentiate customer currency at customer master. As whatever, customer currency you maintain in customer master-sales data, that becomes the base for your document currency.
No doubt, as suggested by Lakshmipathi Sir
you need to change the condition record simultaneously as otherwise, system will consider the currency from condition record if you check in condition tab of sale order.
condition record currency is really a crucial factor, which generally overlooked during conceptualize these kind of requirement.
With best wishes for new year 2013,
JP