He can withhold the funds for 45 days, what he's asking them not to spend. By then…..

https://budget.house.gov/publications/report/impoundment-control-act-1974-what-it-why-does-it-matter

Put simply, if the President wants to spend less money than Congress provided for a particular purpose, he or she must first secure a law providing Congressional approval to rescind the funding in question. The ICA requires that the President send a special message to Congress identifying the amount of the proposed rescission; the reasons for it; and the budgetary, economic, and programmatic effects of the rescission. Upon transmission of such special message, the President may withhold certain funding in the affected accounts for up to 45 legislative session days. If a law approving the rescission is not enacted within the 45 days, any withheld funds must be made available for obligation.

A 2018 Government Accountability Office legal opinion holds that if the President proposes a rescission, he or she must make the affected funds available to be prudently obligated before the funds expire, even if the 45-day clock is still running. This means, for example, that the President cannot strategically time a rescission request for late in the fiscal year and withhold the funding until it expires, thus achieving a rescission without Congressional approval.