Dear Prakash,
The assessee can claim relief u/s 89(1) because he is pushed in the higher slab rate or may end up paying more tax because more than 12 months salary is being charged to tax.
Relief for arrears of salary is computed as below:- (rule 21A(2))
1.Calculate the tax payable on total income ,including the additional salary of the relevant previous year in which it is received.
2.Calculate the tax payable on total income,excluding the additional salary of the relevant previous year in which it is received.
3.Find out the difference between tax as calculated in step 1 and tax as calculated in step 2. (SAY A)
4.Compute the tax on total income after including the additional salary in the previous year to which it relates.
5.Compute the tax on total income after excluding the additional salary in the previous year to which it relates
6.Find out the difference between tax as calculate in step 4 and tax as calculated in step 5. (SAY B)
7.Relief = tax at step 3 - tax at step 6 (A-B)