Replacement of goods under warranty before of GST

We are into business of submersible pumpset , if suppose we have made sale of 2 pumps on dated 10/06/2017 to dealer and he sold to customer , now on dated 01/06/2018 pump is not working , so customer given back to dealer and delear is asking for replacement from us , how we make entry , and wher to show entry in GST