A Khata is simply an account of property details such as owner's name, size, location, area, built up area, etc. It determines the official tax payer for the apartment or property. The details of the khata is to be verified by government officials upon which property owners receive Khata and PID numbers for their respective properties. The Karnataka Government, with an emphasis on Bangalore, has been in the process of computerizing land related records for almost 7 years now and the khata is therefore termed as e-khata. It is totally possible and very common for the government officials to ask for money for the verification process. You have the option of filing a complaint before the Anti-corruption Bureau. Any person can approach and submit a complaint to the nearby ACB police stations at city and district level. The list of the police stations is provided here http://www.karnataka.gov.in/acb/Pages/Contact-Us.aspx. You must submit a written complaint and there is no requirement for payment of any stamp fees or any other payment for making a complaint. I would like to inform you that to file a complaint and for the Anti-Corruption Bureau to take any action you will have to identify the government official who received the bribe and also have proof that the bribe was received by the concerned officers. It is often very difficult to show that and this is the reason that many corruption cases fail at the trial stage itself. You may have to bring the builder as a witness as he was asked to pay the bribe. It will always be better if there is some more evidence available to prove that the bribe was paid. On paper the extra money you have paid is for the e-khata and documentation which may not be enough to prove that money was paid to the government official. Make sure you have sufficient evidences and witnesses to substantiate your complaint when the mater is presented to the Court.
If you are convinced you do not want to pay then contact a good lawyer, send a legal notice to the builder and then initiate legal proceedings in Civil & Criminal courts against the builder and his firm