You create record or secure the records for the cancellation of the booking as evidence.
Issue a legal notice intimating the cancellation of the booking to the builder and demand full refund of the consideration amount given to him so far.
Let him give a reply or comply with the demand made, if he is not responding then you may drag him to the consumer forum for claiming the refund as well as the compensation for the mental stress he caused to you on this aspect.
You can file a money recovery suit also instead of approaching the consumer forum but it will take long time.