If the property is not constructed before the end of the financial year deduction under section 80C can not be claimed for repayment of principal in that financial year.
The reason being this deduction under section 80C is available only in respect of "Residential House Property".
The property which is under construction is not a "House Property" under the tax laws.
Thus deduction can not be claimed in respect of house property which is under construction.
Though your payroll has given you a tax benefit, in case your case is selected by tax department for scrutiny (chances of which are very rare) , the deduction, which is wrongly claimed, would be definitely disallowed.
Further, your employer would also be made to pay penalty for allowing incorrect tax deduction.
