In feus granted after the commencement of this Act the annual feu-duty shall be a fixed amount or quantity, and no casualties shall be payable to the granter of the feu or his successors in the superiority, and it shall not be lawful to condition or stipulate for the payment of any casualty, but it shall be lawful to condition or stipulate for a permanent increase or reduction of the feu-duty, provided that the amount of such increase or reduction shall be certain and that the time or times, from and after which such increase or reduction is to have effect, shall also be certain and not dependent upon any event or occurrence, except the occurrence or recurrence of the time or times at which, under the terms of such condition or stipulation, the increase or reduction of feu-duty is to have effect.