Can you check your calculation again?
1) I do not think HKD did go below 7.75 (HKMA has set an upper limit for HKD of 7.75)
2) There should not be any FX gain/loss on Sep provided that your record FX rate and revaluation rate are the same.
Intuitively, one of the transaction should have FX gain and the other should have FX loss as the FX exposures of these two entities are opposite(e.g., AVC HK has short USD exposure, but AVC VC has short HKD exposure)
Best Regards,