Where -
(a) in respect of any period a designated person or deciding officer has granted supplementary welfare allowance to or in respect of a person and infectious diseases maintenance allowance, including any increase thereof, subsequently becomes payable to or in respect of that person in respect of the period (or part of that period) for which supplementary welfare allowance was paid, and
(b) the supplementary welfare allowance is in excess of the amount which would have been granted to or in respect of the person if the infectious diseases maintenance allowance, including any increase thereof, had been paid during that period,
the excess supplementary welfare allowance shall be treated as payment on account of that allowance.