1. you are right. the bank demand draft is issued in the name of seller simultaneously at the time of registration
2. you need to talk and confirm with the buyer's lender bank
3. also have a clause recorded in your sale agreement that the last instalment of purchase money will be paid after registration within ____ days of registration and if the buyer fails to so pay then the agreement will stand terminated automatically and seller will not be liable for the stamp duty and registration fee paid by the buyer.
4. also write that in the event of aforesaid termination, seller will be entitled to deduct 10% from the sale price and forfeit the same and refund the balance without interest to the buyer. Also buyer will have to execute a cancellation agreement with the seller and all expenses will be borne by the buyer and on failure thereof, the said expenses can be recovered from the purchase money so far paid by the buyer
5. also state that possession of flat will be handed over only on full and final payment. Till then possession will remain with you
6. tell the buyer to incorporate the aforesaid clauses in the sale agreement