GST Law on Defective Goods' disputes between Buyer and Seller

GST Invoice Goods of Rs.10 lacs sold. Buyer says goods are defective but do not return the defective goods to seller and propose to settle the said invoice for 5 lacs. Seller not agreed to settlement and refused to issued Credit Note. Buyer took ITC on total value of Rs.10 lacs. Ques. A : What are the remedies in GST Law for Buyer to avoid the payment of Rs.5 lacs for defective goods ? Ques. B : Whether Seller can also claim his entire amount of Rs.10 lacs as buyer has claimed full ITC on Rs.10 lacs of goods. Pls. Reply as per GST Law.