Tax exemption benefit under 80G 12A

First of all deductions under Chap Vl A are allowed to be deducted from Gross total income then tax liability is calculated on such net income.
That means suppose if ur GTI is Rs 10 lakhs and Rs 25000 is allowed as deduction u/s 80 G then it helps reducing tax liability on directly Rs 25000.

Coming to your question..
Deduction amount will not reduce directly your tax liability from Rs 1 lakh but will reduce that gross income on which 1 lac tax is calculated.