"Imprest" and "Debenture."

When a person fulfilling any employment under any of the Government Boards has occasion to draw "money on account," an "imprest," addressed to the pay-master under that Board, is issued for the required sum; but when the final payment is made upon the "closing of the account," the "debenture" takes the place of the "imprest." Out of what verbal raw material are these words manufactured? I know of no other use of the word "imprest" as a substantive; and though we see "debenture" often enough in railway reports, I cannot perceive the analogy between its meanings in the two cases.