15 November 2013
Do not commit any error in quoting the PAN or other details in the online Form as there is no online mechanism for rectification of errors. For the purpose of rectification you are required to contact Income Tax Department.
The buyer needs to approach income tax department for correction in challan.
He shall have to do it as provided here: http://www.incometaxindia.gov.in/archive/LeftMenu_ChallanCorrectionMechanism_26082011.pdf
With regards to buyer's request for refund, in no circumstance should you agree to any such request. Letting the amount reflect in your Form 26AS would mean that you have sold a property and income tax department shall have every right to question you.
Make it amply clear to the buyer that he needs to get the challan details corrected.