My friend's Russian father lived in Serbia with his mother when he was a young child. He was entitled to a wage in rubles - he was employed by the Russian government - and he had a nice little triangle going. The Russians had pegged the ruble to the US dollar (or close enough) to avoid looking weak, so the ruble was artificially inflated. Yugoslavia didn't do that, so it had a much weaker currency. He would make his paycheck go ruble > USD > yugoslav currency (can't recall the name) and he made quite a bit of 'free' money that way.