Gst effect & treatment of interest charged by our customer due to payment made before credit days

As per sale terms, credit adys say 90 days.

Customer paid the amout 60 days

So customer deduct 10% for 30 days as TOC.

Our Bill price Rs 10000 and GST 18, so total Invoice price 118. 

Now customer charged interest following manner 

10000*10%*30/365 = 82.19

Query:-

Customer should charged interest on basis valie i.e. 100

Also GST on Interest amount 82.19 should be charged