By the last quarter of the financial year, employees must provide documentary proof for exemptions and deductions that they had declared at the beginning of the financial year. This is called proof of investment.
As per the law, employers need to calculate income tax, deduct TDS and remit it every month. Employees can save tax through many exemptions and deductions. But, this is allowed only against proper proof of investments.
The tax deducted in March is based on the documents and not declarations. The employer is bound to disallow any claims that are not backed by proper proof.