Best Answer - Chosen by the Asker
Usually they dont take a comission, but to be proved on it dont hesitate asking. In Russian it sounds:
- Вы берете комиссию? (Viy biriote komisyu?)
There is another sort of fraud is widespread in Moscow. Declared exchange rate is not practised. If you asked 'Why did you exchange using another rate?', they answered 'Declared rates are only for sums more then 1000'. So dont hesitate asking operator the real exchange rates.