Good morning, thank you for the great question.  Because of the Time Statute 31 U.S.C. § 1502 you will need to use FY18 funds. Your finance office should know the procedures and forms you will need to follow/fill out to request an Obligation Adjustment Requirement if your organization still has FY18 funds.  While these can't be used for new requirements, they can be used for adjustments on work during FY18,  specifically for the purposes that your scenario mentioned.  If your finance office does not have FY 18 funds they should be able to help you find an alternative solution.