Payment calculated on the basis of the final reference values used to apply the price variation clause shall be made no later than three months after the date on which these values are published.
Where the final reference values are not known on the date on which an advance payment or final partial payment is due, the purchaser shall make a provisional payment on the basis of the last known references. The parties may nevertheless agree to calculate payments, including the balance of the contract, on the basis of provisional indices.