Actually there is only one field for the exchange rate of a currency, but I need one for sale and another for purchase, then for sale invoices I want that the sale rate is taken, and for purchase invoices the purchase rate.
is there any configuration or module available for this?
maybe a weird question, but why do you want to use different rates? OpenERP has the option of giving a conversion rate at a time, and you can set different rates for different days if needed.
for the same reason exchange houses manage different rates for sale/purchase, for profit.
or because not every partner manage exactly the same exchange rate.