Previous reply was erroneously posted without completing my reply. Read this in addendum to the same.
ST circular was issued for such book adjustments not CESTAT decision which can be reffered.
Further, the GST applicability on such financial credit notes has to analysed case to case basis. The reason for issuance of such credit note plays a vital role to take decision whether the same is treated as income i.e. outward supply for GST purpose.
If such credit note is issued for post sale discount and the same was established in terms of agreement entered into at or before the time of such supply, then such financial credit notes can't be treated as income to recipient since discount is not includible in taxable value. However you should be in a position to substantiate with documentation that the financial credit note has been issued for discount for taking this stand.
If the credit note has to been issued for doing anything or achieving anything which is been agreed between the supplier and the recipient the department may take a stand that such credit note is consideration for supply of services as per Schedule II - Sl.no.5(e) of CGST Act, 2017.