This response is based on the information provided.  We suggest you discuss with your contracting team, program manager, financial team and/or legal department as appropriate. 

Given your team used the funds properly, i.e., for their intended purpose within the time allowed for their use, extending the time of performance is allowable without violating financial laws and regulations.

The requirement was awarded and the requirement still exists so you can extend the delivery date.  Even though the funds will have expired, as long as they remain on contract (until cancelation, five years after expiration) the contractor can invoice and DFAS can pay from those funds. 

If the contract is in the fixed price family, you need to seek some consideration for extending the period of performance.