160Alteration of agreements where duty altered

160  Alteration of agreements where duty altered   If after any agreement is made for the sale or delivery of excisable goods duty paid any alteration takes place in the duty collected affecting such goods before they are entered for home consumption then in the absence of express written provision to the contrary the agreement shall be altered as follows:  (a) In the event of the alteration being a new or increased duty the seller after payment of the new or increased duty may add the difference caused by the alteration to the agreed price.  (b) In the event of the alteration being the abolition or reduction of duty the purchaser may deduct the difference caused by the alteration from the agreed price.  (c) Any refund or payment of increased duty resulting from the alteration not being finally adopted shall be allowed between the parties as the case may require.