Can we cancel Invoice of Service Tax now & Refund amount recd?

Dear All,

 I have a query regarding the Cancellation of the Invoice Dated 31st March 2017 issued in the Service tax regime in the Month of Jan 2020 ( Regime of GST).

Party has paid us 65% upfront payment at the time of booking and thereafter hadnot taken any services and dint paid as well. But now they are active and asking to Cancel the invoice issued and refund the amount of 65% which has been paid already by him.'

Can we decline the refund and Cancellation of invoice since its already 2.5 Years ??

Please inform with the relevance to the Service tax and GST Rules.

Thanks & Regards

Esha Agarwal