Just to add a little to Safari's suggestion. My home currency is HKD but I have Amazon accounts in CAD, EUR, GBP and USD. I can reconcile the year end holdings in each foreign currency by getting the statement from Amazon, however, the balance sheet at year end in HKD shows a variance and needs to be adjusted to reflect the exchange gain/loss from Amazon.
When I initially tried to make a journal entry that only affected the home currency I couldn't because by virtue of selecting the bank this also meant that any entry affected the CAD balance (reconciled, no need to adjust) AND the HKD valuation.
A quick call to QBO provided a neat hack which is to create the journal entry in your home currency, select the foreign currency bank but assign the currency exchange rate to 999999 (on this transaction only!) which will have the effect of debiting (or crediting) to the exchange gain / loss account the amount in HKD but not affecting the bank balance in the foreign currency since it would take 999999 movement in HKD to adjust the CAD value by 1.