Bob:

As an accountant, I couldn't have said it better myself.

Susan

As an example

$1000 USD @ 1.3 = $1300 CAD
At month end you revalue it at 1.28 = $1280. Now you have an unrealized gain of $20 (you will have to pay $20 fewer CAD to settle the account)
Then the vendor gives you a credit note for $100 USD @ 1.27. Now you have a realized gain of $3 plus an unrealized of $18
You pay the balance owed at 1.27. Now you have an additional realized of $27 and no more unrealized