You would be eligible to claim the input in the tax period in which you have received the Tax invoice and do not have to wait until the installment payments. The 6 month time specified in the Law for honoring the payment is from the agreed date of payment (i.e installment dates) and not from the date of the invoice. So as of the invoice date the requirement for "Intention to pay" would be ideally satisfied. Consequently if you fail to pay within the agreed installment date and 6 months from thereon, the input VAT claimed initially will have to be reversed.