Inside of Belarus, you can get Belarusian rubles BYR but NOT always US dollars or euros from automatic bank machines for standard types of credit/debit cards, and you can change US dollars and euros into Belarusian rubles at many exchange kiosks in big railway stations and centers of big cities. Note that you will need to show your passport when exchanging Belarusian rubles back into dollars or euros. Most larger supermarkets, stores and hotels have credit card terminals, but smaller shops often do not. Visa and Mastercard are accepted, while American Express is not.
If you live in the UK or another country where the currency isn't US dollars, Euros or Russian roubles, you should buy some US dollars before you leave as you will most likely not be able to exchange your local currency inside Belarus. Similarly, you won't be able to pay by card or take money out at an ATM and will need to go to a bank with proof of ID to withdraw money from a card. You will be charged a commission of 1-2 per cent.
Converting Belarusian rubles back into hard currency outside of Belarus is extremely difficult except in Lithuania, Latvia, Moldova, strangely enough. However, if you exchange all your rubles before leaving, any last-minute purchases or fines for overstaying, customs, whatever would have to be paid in dollars/euros.
Be very careful: exchange kiosks will not exchange any bill that is damaged or marked. About half of the bills you currently have in your wallet will be rejected for exchange in Belarus. Be sure to take only relatively new and undamaged foreign money with you.
Prices are typically much lower than in Western Europe, especially for supermarket food and service industry. Although 1 US dollar is equivalent to BYR10150 in practice about BYR3000 will buy you one US dollar's worth of locally produced food or goods. Foreign imports, though, will often be closer in price to what they sell for in their country of origin. Although it is possible to find hotels that are cheaper than western Europe, the quality may be significantly lower too. For a western standard hotel you'll have to pay western prices and quite often a lot more than you would pay in neighbouring Poland. For a western restaurant-standard meal, you will have to pay comparatively large sums of money which may sometimes be as high as western prices. A cheaper alternative is a local cafe or столовая stolovaya, self-service cafe/dining room, where you can get a meal for only USD2-3 .