All expenditures must be incurred before the closing date of the sponsored project. This means that the purchase must be complete, and the items delivered and accepted. An unfilled requisition cannot be filled after the final date of the award.
Purchases typically should not occur during the last month of the project unless they are items for service needed during that month. Unspent funds belong to the funding agency. Auditors often disallow purchases made during the final weeks, based on the assumption that the purchases were not actually needed for the project, but that the PI is simply using up the money. We encourage you to purchase in a timely manner, and be certain that the purchase is in accordance with the budget as well as being wholly and completely dedicated to the project.