Oh wow, so you can extend it. I have no idea that you could do that :) Thanks so much that is really good news. The endless debate about where to go in October for a border bounce can be delayed for another 30 days :D I don't know where I got the idea that it couldn't be extended!
I'm still curious about this. How is the conversion meant to be proved? Showing the XE rated printed out? Asking your bank to print and email you a statement in USD or EUR if this is even possible?
I'd just go to immigration and talk to them early as possible to set your mind at ease - I can't see why it should cost more - maybe some offices are a bit strict but it isn't unusual for it to happen. It's good to remember to check they stamp the right thing but coming through we don't always remember. I forgot last time but it was fine. Good luck!