Is it permissible to sell dollars to individuals via a bank cheque deposited into a dollar account, which cannot be withdrawn except when traveling outside the country, at which point it is withdrawn in its equivalent in the local currency of the country being traveled to?

Paying a sum of money to the bank for it to deposit an equivalent amount in dollars into a person's special account is considered sarf (currency exchange). Sarf requires actual or constructive possession (qabd). One form of constructive possession is the bank entry of a sum of money into the customer's account. If the bank directly deposits the dollars into the person's special account upon receiving the equivalent amount, then it is a valid sarf, and its validity is not affected by the fact that the account holder cannot withdraw it unless he travels outside his country.