In Tibet, the only place to exchange foreign currency and travelers cheques is the Bank of China. The top end hotels in Lhasa have exchange services but they are only available to guests. The sensible thing to do is to change as much money in Lhasa as you think you need. The only other places to change money are Xigaze and Zham. If you are traveling upcountry, try and get your cash in small denominations: RMB100 and RMB50 bills are sometimes difficult to use in rural Tibet. If you don't like the idea of turning up at the border without Chinese currency you can buy cash RMB from banks in Hong Kong and the Bank of China in large cities.
Credit cards are not recommended as a way of payment in Tibet. Flights and Lhasa hotels cannot be paid using credit cards. The Lhasa central branch of the Bank of China is the only place in Tibet which provides credit card advances. A 4% commission is usually deducted and the minimum advance is normally RMB1200.
Traveler's cheques are now acceptable at the Bank of China. It is recommended that you cash them in Lhasa.
Remark: In Tibet coins are not acceptable even for tipping and sometimes the new version of paper currency is not recognized and not accepted.
(Tibet Travel Expert)