Sir firstly send a legal notice to buyer asking compensation and damages for delay along with mental harassment caused due to his deficiency in service. if on service of notice within 15 days builder donot pay your asked damages file a compliant before the Consumer Court for deficiency in services by builder the court shall grant you damages for delay along with compensation for harassment.
There are many number of cases where the consumer court has awarded significant amount on delay by builder to the consumer/ buyer and also cancelled the sale and builder has to refund amount with the interest.