with such missiles in economic order quantities when cost savings are achievable. (c) Condition for out-year contract payments A contract entered into under subsection (a) shall provide that any obligation of the United States to make a payment under the contract for a fiscal year after fiscal year 2024 is subject to the availability of appropriations or funds for that purpose for such later fiscal year. (d) Mandatory inclusion of pre-priced option in certain circumstances (1) In general If the total base quantity of Sentinel intercontinental ballistic missiles to be procured through all contracts entered into under subsection (a) is less than 659, the Secretary of the Air Force shall ensure that one or more of the contracts includes a pre-priced option for the procurement of additional Sentinel intercontinental ballistic missiles such that the sum of such base quantity and the number of such missiles that may be procured through the exercise of such options is equal to 659 missiles. (2) Definitions In this subsection: (A) Base quantity The term base quantity means the quantity of Sentinel intercontinental ballistic missiles to be procured under a contract entered into under subsection (a), excluding any quantity of such missiles that may be procured through the exercise of an option that may be part of such contract. (B) Pre-priced option The term pre-priced option means a contract option for a contract entered into under subsection (a) that, if exercised, would allow the Secretary of the Air Force to procure a quantity of intercontinental ballistic missiles at a predetermined price specified in such contract. (e) Limitation The Secretary of the Air Force may not modify a contract entered into under subsection (a) if the modification would increase the per unit price of the Sentinel intercontinental ballistic missiles by more than 10 percent above the target per unit price specified in the original contract for such missiles under subsection (a). (f) Modifications to the Intercontinental Ballistic Missile Site