Flat acquisition

I recently agreed to purchase a flat in Delhi. Initially, an agreement was made with some advance amount paid. After a few days I paid the rest of the amount. On the date of registry, the sale deed was not registered as we came to know that the registration process had been stalled. It is to be noted here that the deed papers had been signed by me and the seller both. However, the seller kept the deed with himself and I was only allowed to take photos of the deed. Now we came to know that there is a severe problem of water supply in the flat. So we told the seller that we were not interested in buying this property anymore. He denied returning the money I paid. Instead, he offered to sell us another flat at a new deal. So my questions are(1)Can we legally buy this new flat instead of the previous one at this stage?(2)How should the previous deed be superseded/cancelled?(3)What are the legal complications that may arise due to this?(4)Will I be charged for stamp duty and registration again?