The president cannot unilaterally remove parts of an appropriations bill signed into law.
Title X of the Impoundment Control Act of 1974 gives the president the power to suggest rescissions to budgetary items within appropriations bills. However, these suggested rescissions must be approved by Congress to take effect. If Congress rejects a suggested rescission or fails to approve a suggested rescission within 45 days the funds in question must be made available.
A 2018 decision by the Government Accountability Office prevents the president from strategically suggesting a rescission late in the fiscal year to delay the disbursement of funds until they expire.
The Constitution gives Congress the exclusive power to appropriate funds for government spending. Historically, except in certain emergency circumstances, the president has not made spending decisions without congressional approval.