From an accounting standpoint, the fiscal year in which a purchase is charged to is determined by the date the goods/services are actually received--not the date which the payment is made. Because the date which the goods/services are received is also the date which the liability is incurred, this is the date in which the expenditure is recorded against the budget.
Administrators should plan accordingly and well in advance to ensure the appropriate timing of expenditures occurs in conjunction with budget availability.