If your client has mistakenly entered the TAN number instead of the PAN number while paying the TDS challan online and the amount has been debited from their bank account, here is what they can do:
1. Contact the income tax department's e-filing customer care immediately and inform them of the mistake. Provide them with the details of the transaction such as the TAN number and the amount debited.
2. The income tax department may ask your client to submit a written request for rectification, along with a copy of the TDS challan and proof of the mistake (such as a screenshot of the incorrect TAN number entered).
3. After verifying the details, the income tax department may rectify the mistake and credit the TDS amount to the correct PAN number.
4.Your client should also check the status of the TDS credit in Form 26AS, to confirm the credit is reflecting under the correct PAN number.
5.If the mistake is not rectified even after following the above steps, your client can file a refund claim for the TDS amount debited from the wrong TAN number.