cancellation of booking in real estate

You can go for Refund provided that the sale should happen with in 6 months prior to appointed date.

The cancellation has been made with in 6 months after appointed date..

You can prepare Credit note and proceed for Refund as per Sec.142 of CGST act.

Hope that your customer is Unregistered iin GST regime, If he is registered, then the process is different.

Now you file the service tax return and show the excess amount paid in advance calloum and apply for refund in concern department.