An advance shall be made available to each imprest administrator, the amount of which, fixed by the instrument setting up the imprest account and, where applicable, revised in the same manner, shall not exceed one quarter of the foreseeable amount of annual expenditure to be paid by the imprest administrator. The instrument setting up the imprest account also lays down the conditions under which, exceptionally and with the assent of the assigning public accountant, the advance may exceed this maximum. The advance is paid by the assigning public accountant at the request of the administrator, approved by the authorising officer.