Grants Accounting - awards in foreign currency

 I'd be interested to hear how people manage grants that have been awarded in other currencies than the functional currency and exchange rate fluctuation. 

We can only hold budgets and expenditure in GBP but because of the fluctuations in exchange rates we are either spending more than the budget or under-spending.  Invoices can obviously be raised in the foreign curency but the exchange rate at that time might differ from the exchange rate at the time the budget was created and converted to GBP.