Reversal of Pre GST invoices

Hey Guys,

Need your views on this

Situation:
Services completed and invoices raised in pre GST period and shared with customer in pre GST for booking on which service tax has been paid by the service provider
Now service recipient has rejected the invoice in entirety in post GST period and has asked the service provider to raise the invoice as per new GST law

Query:

1) What is the best way to handle this situation?
2) What will happen to service tax already paid on those services?
3) In case of reversal of Pre GST invoices under post GST period how to show the same in return?