How to treat the transaction for which Credit Note issued and also Purchase Invoice Received?

Please tell the treatment of the below mentioned transaction in Accounts and its implication in GST:

Mr. A, the Supplier sold 100 goods to Recipient, Mr.B. Out of them few items, say 20 goods were not up to the mark, hence Sales Return was to be made.

Now, Mr. A had issued Credit Note to the Recipient, Mr.B as it was Sales Return for those 20 goods and accordingly adjusted its output tax liability in the month in which CN was issued. But, later it is found from the GSTR-2A of the Supplier (Mr.A) that, Mr.B had again showed in his GSTR-1 treating those 20 goods were sold to Mr. A. 

So, how to treat this transaction? And in some cases, Mr. A had cliamed the ITC also inadvertently, that means he had reduced his Output Liability by issuing Credit Note and again he claimed the ITC from the GSTR-2A uploaded by Mr.B. What shall be the possible solution to rectify it?

Thanks