As facts and context of the query is not clear, I am taking liberty to presume certain things to share my views.
I am presuming that said value reduction is nothing but post-sale discount agreed between buyer and seller. And, if so, my views are as under:
Section 15 deals with 'Value of Supply' and sub-section (3) reads as follows:
"The value of the supply shall not include any discount which is given––
(a) before or at the time of the supply if such discount has been duly recorded in the invoice issued in respect of such supply; and
(b) after the supply has been effected, if–
(i) such discount is established in terms of an agreement entered into at or before the time of such supply and specifically linked to relevant invoices; and
(ii) input tax credit as is attributable to the discount on the basis of document issued by the supplier has been reversed by the recipient of the supply."
If it is post-sale discount which can NOT be established (another presumption of mine, as query is not clear) "in terms of an agreement entered into at or before the time of such supply and specifically linked to relevant invoices", then, buyer is within its right to raise 'commercial / accounting debit note' (please note that it is not a debit-note u/s 34 of the GST) towards 'Basic Value' towards discount (which does not include GST portion).
Similarly, in above circumstances, seller should raise corresponding 'commercial / accounting credit note' (please note that it is not a credit-note u/s 34 of the GST) towards 'Basic Value' towards discount (which does not include GST portion).
An in such a scenario, neither buyer needs to reverse any ITC nor seller gets any re-credit towards taxes.
And also, question of any output tax-liability against such 'discount' does not arise as there is no supply against subject 'discount'.
These are strictly personal views of mine and the same should not be construed as professional advice / suggestion.