Builder not providing receipts for VAT collection.

Dear Sir I had purchased a flat in the year 2007. The agreement was registered in the month of October -2007 and possession given in the year April-2008.During the handing over of the Flat the builder has taken an undertaking that VAT would be payable by Flat purchaser. First the builder sent us notice to collect the VAT along with interest and penalty.When asked the proof of govt challan for payment of VAT the builder refused the same.Now the builder is planning to send us a legal notice and threatening us with a court case else is asking us to pay the VAT+Penalty+Interest.Please advise