Rather than making up things like stress , just call Hertz and point out that the bank fees have left you out of pocket by $45 and I expect that they will do something such as offer a voucher that you can use

On November 3, the exchange rate from XE shows as 7.75 and for 1st Oct was 7.76 so there would have been a small loss of around HKD7.61 (USD1)

With the exchange rates Citibank uses , it would seem that there could be around a 3% conversion fee being applied on both the purchase and the credit

It isn't Hert's fault that the bank charges the customer this , but I would be surprised if customer relations wouldn't offer something