Cash payment

We are register dealer in GST. Recently we make a Invoice Rs.88,500.00, Dated -10/05/2018 ( Taxalbe Value Rs. 75000.00 + GST13500 = Rs.88,500.00) to a Unregister Dealer . He (UD) made cash paymnet whole amount dated 11/05/2018.

Now our question is-

1) Income Tax Penalty will be charged on whom Our or Unreigister Deler or BOTH

2) How to avoid this penlty  (if party do not want to paid this above amount by cheque)