Attachment of a property can be done only in an EP proceedings and not before that, but you may not be able to attach the flat at that time because the builder would have disposed it by that time so you may not find any property to bring it on sale to recover your money
In a money recovery suit the unregistered sale agreement will be considered as a receipt for the money paid to the vendor, hence the ABJ option is the best one during the pendency of the suit so that the vendor cannot sell the property attached by court or he has to furnish security for the suit amount.
