Several decades ago, it was the suggestion of what was supposedly a reliable travel editor that merchants might hold back depositing charge slips if a particular currency was hving a tough time i.e. say a British merchants took an American credit card at a time the USD relative to the GBP was in a state of decline. This "genius" claimed the merchant would find it advantageous to hold on to the charge for a bit of time waiting for the USD to decline further. Of course people here with a degree of inteligence realize how absurd this was even then as the merchant gets his or hers in their own currency and it makes no difference to them whether GBP 10 becomes USD 1.60 or USD 1.63 or USD 1.70....but that was the advice. Ugh.

Today, 99% of charges are done electronically, are in the merchant's bank account the next day and in the network at the same time so perhaps a day later the charge is posted.

As far as ftf's are concerned, it is not the currency it is where the merchant's bank processor is located. That is why many people who buy airline tickets, even from a US agency which is acting for the foreign airline, get nailed for a ftf on airline tickets on a purchase that they feel never leaves the USA but in reality is processed by the airline's acquirer in the country the airline is located.