commutation deduction

Do you mean tax-ability of pension received in lump sum ....

i.e. Commuted pension received by government employees is wholly exempt from tax.

In the case of commuted pension received by non-government employee, the maximum amount exempt from tax is restricted to the one-third of the total commuted pension, to which employee was entitled, where the employee also receives any gratuity. Otherwise, the exemption is limited to commuted value of ½ of the pension to which employee is entitled.

You can file rectification petition within 4 years from end of the FY from the date of order passed which is sought to be rectified as per subsection 7 of section 154 of the Income Tax Act, 1961. Further once rectification petition is received by the income tax department, the same must be disposed within 6 months from end of the month when petition is received.