194-ia penalty

I did not deduct tax at source u/s 194-IA as the seller was an old lady and I was told she is not liable to tax. She applied for PAN only for the specific sale of property transaction. I have now got a show cause notice from IT dept asking why I have not deducted and instructing me to remit the TDS with Interest and penalty. On enquiring from the old lady , she had filed ROI and shown the sale transaction. If I deduct tax now , it will be to the seller's credit . Now since the transaction is long over how can I recover this amount from her? Capital Gain tax is already paid for the consideration. How can mere non filing of TDS entail me to deduct again for a past transaction which has also been taxed?