When you create a billing record in a currency other than your own, a currency exchange rate is applied to it. The exchange rate used in Ezekia is updated every night automatically.
The value is stored in the chosen currency and converted to USD using the exchange rate on that day. Each day we update the USD value based on that day rate, so the underling chosen currency value never changes, but what you see in your base currency does.
However, we stop converting the underlying USD value on the day the date is reached, so the exchange rate stays static for all future reporting.
You cannot edit the exchange rate, but you can edit the date it was due, to the day the funds reached the bank. This should update the values to reflect the exchange rate. Not ideal as the date might be wrong for reporting.
Another option is to use revenue to report targets and then use invoices to report money paid. You can then set different dates for both, using different daily exchange rates. Indeed, that is how billing is designed, you use revenues for forecasting and invoices for actual money paid.