Excess salary repaid

One of my client received an increment from 2012 and received the amount was received in 2013 as arrears. There after the amount was included in regular salary and tax calculation was done as usual. This year the departmental audit found that the increment given was wrong and asked him to repay the amount which was received from 2012. He also repaid the amount 2,45,000 by DD to the treasury.
1) How to calculate tax liability?
2) What are the possible solutions?