Congress may not grant powers (the Preamble and Amendment X prohibit it).
Or abdicate them.
This can be surprisingly difficult.
It can’t, for example, abdicate its power to legislate by delegating to the Attorney General power to “legislate” by enforcing statutes selectively.
Congress may not grant powers. The Preamble and Amendment X prohibit it.
"The powers not delegated to the United States by the Constitution,
nor prohibited by it to the States, are reserved to the States respectively, or to the people."
Nor may Congress abdicate other powers. For example, just as the Executive may not deny the Senate the use of its power of advice and consent where indicated (see Executive in Conflict), the Senate may not abdicate this power. Whenever the use of a power is indicated, it must be used. Nowhere does the Constitution grant the Senate discretion over whether to use its power of advice and consent. And just as the Senate must file suit against the President if the President denies it its power to advise & consent (see Executive in Conflict), the President must resort to the courts to uphold the Constitution if the Senate abdicates this power when its use is indicated.
The selective use of powers is called discretion (see Judiciary in Conflict). The Constitution identifies where discretion may be used. It may never be used where the Constitution doesn't specify it may be used.
Congress doesn’t have the authority to prevent the Department of Justice from enforcing laws Congress itself passed, even by using its power of the purse. It may make overall funding decisions regarding any branch, but may not pick and choose which federal laws those branches may enact, enforce or adjudicate.
Legislation prohibiting “spending money” on a particular law is not valid because Article I gives Congress full authority to repeal or revoke their own legislation: that is the extent of their power over the execution of federal law.
It is checked by abutting the Executive branch’s power to execute the laws and may not encroach on that branch’s power.