S. 10 (23C): Educational institution-Bogus donation-Donation supported by certificate of recognition of Director General of exemptions and bank statements-Denial of exemption is not proper-Retracted statements not incriminating material-Cannot be used to reopen concluded assessment-Gratuity-Leave encashment-Provision for gratuity or leave encashment-Constitutes ascertained liability-Allowable as deduction. [S. 12AA, 80G, 143(3) 153A]