The Braintree daily summary mail just arrived, this shows the amount as "20,00 € EUR" so there is still a bit of work to do.
This was a payment which I had defined in league as 20 $ USD. I'm not sure how much control you have over this in the package which goes out to paypal/braintree, but I'm guessing that their protocol can handle you specifying both amount and currency. My bet is you're just sending the amount the packet you send to PayPal, and that PayPal defaults to that user's default currency. (And even that is bent at the PayPal end, my default currency should be CHF.)
I'm not saying that you need to handle and convert all known currencies in the plugin so that the user knows exactly what the cost in Drachma will be, but that when the user arrives at the paypal/cc confirmation then they are probably not expecting to have to check or change the currency.
EDIT: Years ago I booked a flight on a carrier's website, and used their link to also reserve a rental car for that trip at a discount. When I arrived at the destination and went to pick up the car the staff told me that my reservation had been for the previous day, and that they had let the car out to someone else, and cancelled my reservation, but that they would be happy to rent me a car at the full rate...
I later tried to get some of the additional costs back, to no avail, the airline said "look at our T&C, you must check the information" the car rental co said "We had a valid contract which you broke.". Both are true, but the fact is that I should have either been able to trust the computer interface to do exactly that which computers do well (and humans less well), or the air carrier site should have just said: "Thanks for your money, if you want a car then it's your problem, we don't do cars".