We are Buyer and deal with seller, now seller refuse to sell house and ask for cancel sale deed

I am writing to seek your legal advice regarding a property transaction that has become problematic. As the buyer, I entered into an agreement with the seller around six months ago. Initially, I made a token payment of ₹20,000. Two to three months later, I paid an additional ₹4,00,000 to secure a date for the dastavej (sale deed) registration. In the meantime, I completed all the loan procedures, and the bank has been deducting interest from my account monthly. However, the seller has repeatedly delayed the process. He claimed to have booked a registration date twice but canceled each time within 20 minutes, which seems suspicious to us. Recently, he informed us of a new registration date, but when we asked for the stamp duty receipt or a copy of the dastavej document, he refused to provide them. He insists that we pay him cash of 20,00,000, as per a subsequent verbal agreement, before showing us any of the required documents. We have expressed our willingness to cancel the deal and requested that the seller return the funds we have already paid. However, the seller is now demanding that we first provide him with the sale deed before he returns our money. Given these circumstances, we are unsure of our legal standing. Is it legal for the seller to withhold important documents and demand further payments before proceeding? What actions can we take to either move forward with the transaction under fair terms or recover the money we have already paid? Your legal guidance would be greatly appreciated.