B2CS sale entry in B2B

You have two options.

Option 1: Amend the B2B invoice by changing the invoice amount & taxable value to 0.01 and GST to 0. AND Amend the B2C figures for that same period (this does Not require any changes or new entries in the Books AND the correct effect is captured in the right month).

Option 2: Raise a Credit Note to unregistered dealer in Books but report the same (in GSTR-1) against the wrong GSTIN (i.e. under Registered Credit/Debit Notes Table) to nullify the wrong reporting. And issue a new invoice to the same unregistered dealer and report it under B2C Table of GSTR-1. (Here you need to issue a credit note and new invoice for B2C sales in the books in the current month, the effect of these will be captured in the upcoming month/period of your GSTR-1, but they will cancel out each other, thus no effect on output tax).