Section 15 CGST Act

(1) The value of a supply of goods or services or both shall be the transaction value, which is the price actually paid or payable for the said supply of goods or services or both where the supplier and the recipient of the supply are not related and the price is the sole consideration for the supply.
(2) The value of supply shall include–––
(a) any taxes, duties, cesses, fees and charges levied under any law for the time being in force other than this Act, the State Goods and Services Tax Act, the Union Territory Goods and Services Tax Act and the Goods and Services Tax (Compensation to States) Act, if charged separately by the supplier;
(b) any amount that the supplier is liable to pay in relation to such supply but which has been incurred by the recipient of the supply and not included in the price actually paid or payable for the goods or services or both;
(c) incidental expenses, including commission and packing, charged by the supplier to the recipient of a supply and any amount charged for anything done by the supplier in respect of the supply of goods or services or both at the time of, or before delivery of goods or supply of services;
(d) interest or late fee or penalty for delayed payment of any consideration for any supply; and
(e) subsidies directly linked to the price excluding subsidies provided by the Central Government and State Governments.
Explanation.––For the purposes of this sub-section, the amount of subsidy shall be included in the value of supply of the supplier who receives the subsidy.
(3) 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.