Where the Minister for Finance is satisfied, with respect to any class of persons in receipt of any salary, fees or emoluments payable out of the public revenue, that such persons are obliged to lay out and expend money wholly, exclusively and necessarily in the performance of the duties in respect of which such salary, fees or emoluments are payable, the Minister for Finance may fix such sum as in that Minister's opinion represents a fair equivalent of the average amount for a year of assessment so laid out and expended by persons of that class, and in charging the tax on such salary, fees or emoluments, there shall be deducted from the amount of such salary, fees or emoluments the sums so fixed by the Minister for Finance; but, if any person would but for this section be entitled to deduct a larger amount than the sum so fixed, that sum may be deducted instead of the sum so fixed.