Let me cut the story first, I will go into detail if anyone finds the summary vague.

On Oct 1, I arrived at PDX at 1830. Went to Gold member board and could not find my name, so I went to the booth and got it fixed.
Leaving PDX at 1926 happily and had the car returned on Oct 6 4pm at PDX.
Got a receipt, checked and agreed the charges to original rental agreement, case closed to me.

Then I continued my travel to LAX, started another contract on Oct 6 6.30pm and closed it on Oct 11 and I headed back to HKG.

Then on Oct 15, I got an e-receipt saying I just returned my car for another rental started on Oct 1 1830 and I got charged with USD761, equiv. to HK$6,050 at 7.9.

Case reported to HK Hertz and forwarded to US Hertz.
Follow-up is done and a credit in USD761 is made.
Yet the credit is exchanged at 7.5 to HK$5,700.

The bank says the prevailing rates for debit/credit are naturally different, go for merchant.

HK Hertz says this is nothing deal with them and nothing they can do, fair enough.
US Hertz says refunded is made and not a penny is short and nothing they can do either.

Where should I go?
Any thoughts dear FTers?