GSTR-3B Vs GSTR-1

Dear all Good Afternoon,

My Query is : Now we are filing GSTR-9 for the FY 2018-19. During the period the client declared sales in 3B under IGST (Interstate) but in GSTR-1 it is local state sales. the difference is showing in CGST & SGST columns as per GSTR-1. the department also issued DRC-01 and proposed tax for that difference. please suggest.

thanks & regards