In case of Debit Note: Reversal of Output liability is more than output tax liability under GST

Dear Members or reader of CCI,

I would like to inform you that, I have one case, in which the client sale flat and charged GST thereon @ 12% in the FY- 2018-19 but in the month of April 2019 the client raised credit note for such sale and reverse output liability @ 12%, further the client re-sale that flat to another customer and charged GST @ 5% (not eligible to claim ITC), now the problem is GST levied on Credit note is much more than GST charged on Tax Invoice issued in the month of April 2019, because till the September 2019 the client neither filed their GSTR-3B return due to this nor any GST output liability to adjust GST levied on Credit Note, which was carried forward from the month of April 2019, Please provide your  legal opinion to do in this case.

Thanks in advance..