(1) Where an instrument is chargeable with an ad valorem duty in respect of

(a) moneys in a foreign currency,

(b) a stock or marketable security, the duty shall be calculated on the value of the money in Ghana currency according to the prevailing rate of exchange on the day of the date of the instrument or in the case of stock or security, according to the average price of the stock or security.

(2) The Commissioner, if not satisfied with the evidence, is not bound to assess the duty in conformity with subsection (1).