They can't use $ from a 2017 budget that was spent to pay for FYE 2019 expenditures. If an agency doesn't have a budget, it can't SPEND $ (that's why "essential" employees are working without pay.)
https://www.washingtonexaminer.com/policy/economy/federal-contractors-are-hardest-hit-by-shutdown-because-they-wont-get-back-pay