If the contractor submitted a valid invoice for work accomplished on an existing contract that has not been legally closed, the contractor is entitled to payment for that work. If you are unsure about the validity of the bill, check with your local legal department prior to paying it. They can determine if the invoice is valid and the contractor is entitled to payment.
Assuming it is a valid bill – FY18 3080 funding (Other Procurement Air Force) was current and available for new obligations in FY18, FY19, and FY20. Following that period, it went into expired status on 1 October 2020 (FY21) and will remain in expired status for a total of five years. During the Expired Phase, the funds retain their identity (Fiscal Year, Appropriation, and Program Element/Budget Line Item) and unobligated amounts are controlled at the Service or Command level.
Because the appropriation account cited on the existing contract is expired, the payment must be made with expired funds, using the same fiscal year funds and appropriation account that were cited on the original contract. In your case, FY18 3800 funds (OPAF) with the same identity,can be obligated for upward adjustments for in-scope work of the existing project. If funds are not available in the original fund citation to cover the upward obligation adjustment to that budget line item, a funding realignment between programs within that fiscal year appropriation account would be done. The level of detail required to be maintained for expired accounts is the same level that is required for current accounts in order to ensure proper administrative control of those appropriations. After the realignment is done and there are sufficient funds in the original budget line item, the obligation would be made against the originally cited line.
The originally cited appropriation account is used to pay that old bill because such funds are still available for necessary upward or downward adjustments to the original obligation. An easy way to remember it: if the paying office receives a valid invoice anytime during the period the original cited appropriation is considered currently available plus the five year period that the appropriation is considered expired, the payment must be made out of the originally cited appropriation account.
The Financial Management Regulation (FMR), Vol 2A, Chapter 1, Paragraph 010107, Section B25, states, “Expired Appropriation. An appropriation whose period of availability for incurring new obligations has expired but the appropriation is not closed (canceled). During this period, the appropriation is available for adjustment to, or payment of, existing obligations.”
Recommend that you also discuss with your local Comptroller on their local policy.