Query regarding appeal filed to income tax?

While filing appeal to income tax i have not purchase stamp paper on the date of filing appreal as letter of authority. ( I am tax practioner)

After completing  the 2st hearing (2 years after the filing of appeal)  regarding this case Comissioner of Income Tax ask me to show letter of authority. I have not purchase Stamp paper on date of filing appeal. Now the question arise whether Stamp paper purchased today's date will be valid for back dated hearing to income tax???

Or shall income tax has provision to use "COURT FEE STAMP" on Letter of Authority??????? & same hearing will be treat vaild or legal??