Payment Linked to the Construction Input Index

The contract includes the following clause: "All payments will be linked to the rise of the construction input index as defined at the end of this clause. The buyer commits to pay the seller on time any amount or amounts due according to the indexation terms. Any differences that arise, if they arise, in connection with the indexation will be paid in accordance with the payment dates set in the consideration appendix, along with each payment. Any payment actually made to the seller (by the buyer or on his behalf) will be entitled by the seller, at his sole discretion, to consider and calculate as a payment of the principal amount plus indexation differences, interest, and VAT. If the payment is linked to the rise of the construction input index, it will be paid according to the following calculation: etc.", is this permitted?