NEW DELHI: Suppliers giving post-sale discounts through credit notes under GST will have to ensure that the client gives an undertaking or a CA certificate stating that the Input Tax Credit (ITC) availed on the discount value has been reversed, the CBIC has said. Currently, there is no mechanism to track whether ITC on such discounts has been reversed or not.EY Tax Partner Saurabh Agarwal said these documents, with a unique ID number, serve as proof for audits and ensure consistent application of GST rules on post-sale discounts.