Ability to change the exchange rate of a purchase invoice after it has been authorised

Dear does not let you amend an exchange rate on a purchase order once the invoice has been authorised. If a mistake is spotted / currency is booked after the invoice date, it is impossible to amend the rate. You cannot undo the invoice once stock is sold as this throws an error. Is it possible to allow the user to change the exchange rate after the invoice is authorised?