Proportionate reversal of input tax credit: suppliers must obtain verifiable certificates or recipient undertakings to validate post supply discounts. Suppliers giving post-supply discounts via tax credit notes may exclude those discounts from taxable value only if Section 15(3)(b)(ii) conditions are met, notably proportionate reversal of input tax credit by the recipient. Until portal verification exists, suppliers must obtain a CA/CMA certificate (with UDIN) or, below the prescribed tax threshold, a recipient undertaking, detailing credit notes, linked invoice numbers, ITC reversal amounts and the FORM GST DRC-03/return reference; these documents will serve as admissible evidence for compliance and must be produced to tax authorities when required.