If (1) any processor, jobber, or wholesaler has, prior to the date a tax with respect to any commodity is first imposed under this chapter, made a bona fide contract of sale for delivery on or after such date, of any article processed wholly or in chief value from such commodity, and if (2) such contract does not permit the addition to the amount to be paid thereunder of the whole of such tax, then (unless the contract prohibits such addition) the vendee shall pay so much of the tax as is not permitted to be added to the contract price.