As you would know, this is what the standard says:
A foreign currency transaction shall be recorded, on initial recognition in the functional currency, by applying to the foreign currency amount the spot exchange rate between the functional currency and the foreign currency at the date of the transaction.[IAS 21.21]
So it is not clear about the way credit notes are handled. I think both methods should be okay if you uniformly apply as a policy. But I would prefer the latter as it is easier to follow. ie. you always use the current ex rate for any transaction or reversal of the same and difference is recognized as exchange gain or loss.