Provided that if a resident Indian having purchased a certificate, subsequently becomes Non-Resident during the currency of the maturity period, the certificate shall be Encashed or deemed to be encashed be encashed on the day he becomes a non-Resident, and ab interest shall be paid at the rate applicable to the Post Office Savings Account, from time to time, from such day and upto the last day of the month preceding the month in which it is actually encashed”