There is no law that requires budgeted funds be expended. The only rush to spend budgeted funds is the concept of zero based budgeting where the starting point for a new budget for any Department or agency is what was actually spent in the previous fiscal year. . . the zero base. No bureaucrat wants to be trying for an increase in their budget starting from less than the previous year’s budget amount, so they desperately scramble to expend every penny budgeted, and more, if possible to start the budget amount as high as possible!
The President has the authority, especially in Foreign Affairs, to withhold allocated funds if he deems it advisable to do so, otherwise the House of Representatives would be unconstitutionally in charge of Foreign Affairs by simply ordering spending not approved by the president’s foreign policy.
Actually, yes there is.
States that the "obligation" can be delayed up to 45 days, to allow for a request that Congress rescind the spending obligation. After that window, the Comptroller General may - having given Congress 25 days notice - file civil suit against the appropriate part of the Executive branch to compel the obligated spending.
Problem for the Left: Schiff raised the issue 2 days after the 45 day window, and OMB released the money the next day. Kinda rude of Trump to abuse the allowed 45 day window (he didn't notify Congress of a request to rescind) and held the funds 3 days longer ... but then again, we all know (and approved!) of Trump being rather rude in high-stakes finance, usually to great benefit. Also, Schiff didn't file notice with Congress that the Comptroller General was filing civil suit over the issue, as the law prescribes. Law stretched, but not broken, and House Democrats didn't follow up as they're supposed to over such a matter.