A post from Matt Geddes on Discuss regarding exchange rates suggests that it may be useful to add the ability to define the exchange rate used in a given transaction.
This could be added as a new optional attribute to the transaction/value element, perhaps:
<value currency="EUR" exchange-rate-used="1.43">1000</value>
This is intended to indicate that the exchange rate used differs from the exchange rate of the value/@value-date.
The use case appears to arise from work in Bangladesh, which has identified that many donors have their own exchange rates (often set by their central bank / ministry of finance) and therefore when converting projects between currencies, these are the ones that need using.