In the USA, the Secretary of the Treasury is nominated by the president and approved by the Congress. He/she is not an elected official and has no specific constituency (s/he is supposed to act in the interests of the whole country, and serves until s/he resigns, or the president fires him/her, or until there is a new president).

Chancellor of the Exchequer is member of parliament that has been elected. The Secretary of the Treasury is appointed by the Treasury. He is not elected and he is impartial. For more details and comparisons see: http://en.wikipedia.org/wiki/Treasury