Honble Supreme Court vide its judgment rendered in case titled as Suraj Lamp and Pvt. Ltd. v/s State of Haryana and another held that Transactions of the nature of GPA sales or Agreement to Sell/GPA/Will transfers do not convey title and do not amount to transfer nor can they be recognized or valid mode of transfer of immovable property.
So, you should not proceed with this deal at all. The money paid by you can be recovered by way of a suit for recovery against the Builder even if there is no such provision as the seller has to title to sell in absence of a registered sale deed in his name.