Many years ago a car dealer friend of mine was going to Belgium to buy a car. As I was working in London he asked me to take some Pounds Sterling and exchange them for Belgium Francs. He said better rates were available in London. At the time I think the exchange rate was something like 200 - 250 Belgium Francs to the Pound, and I had £6000 to £7000 to change (it was for a luxury car).
I rang around, and an exchange bureau on the Edgware Road offered a far better rate than every one else. I arrived at this poky little kiosk staffed by some rather tough looking guys who did not speak good English. I said I had come to pick up about 1.5 million Belgium Francs. The rate he scribbled on a bit of paper was not the same as on the phone. Turns out rate he'd quoted me on the phone was the BUYING rate - SELLING rate was worse. I escaped clutching my briefcase holding holding more than a years salary. I had to visit three other places to exchange all the cash. I ended up on the train that evening with a bulging carrier bag full of low denomination notes.