GST paid under wrong head

Mohan Kale (336 Points)

10 January 2020  
A sales bill for inter-state supply (Maharashtra to Gujarat) was raised in May 2019 charging IGST. However, while reporting in monthly GSTR-1 same was shown as intra-state supply and accordingly tax was shown in CGST and SGST. Subsequently, while making payment of GST and filing GSTR-3B same mistake was followed resulting purchaser not availed of ITC. The mistake came to notice in December. My query is how do I correct this error now in terms for correct reporting in GST return as well as payment under correct head?

Thanks in advance.