GSTR 1 and 2B in case of wrong filing by the party - Goods and Services Tax - GST

Dear Experts, I m seeking your help to attain clarity: Consider a situation where Mr. A, a GST-registered vendor, has two GST-registered customers: B (an existing customer) and C (a new customer). During the onboarding process of C, Mr. A mistakenly entered B s GST number in the system for C. Now, let s consider the following: In January, Mr. A did not make any sales to B but made an IGST tax value of Rs. 100 to C. Due to the error during onboarding, Mr. A filed his GSTR-1 under B s name instead of C s. What steps can Mr. A take to correct this mistake in his GST returns? In the same month (January), B s GSTR-2B shows the invoice. Should Mr. A permanently reverse the Rs. 100 in 4B(1), or should he adjust it by reducing Rs. 100 in 4A(5)? Also, if B s total ITC in 2B is Rs. 500, should Mr. A only report Rs. 400, knowing that Rs. 100 of the ITC is not valid for him? Can Mr. A issue a credit note for the above, to correct the error and report it in the CDNR section of his GSTR-1 in the following month regarding B? Suppose Mr. A made an IGST tax value of Rs. 200 to Mr. B. What actions should Mr. A and Mr. B take in terms of GSTR-1 and GSTR-2B reporting and further course of action Regards, S Ram