The Constitution allocates to the president sole authority over foreign policy (short of declaring war or signing a treaty). It does not permit Congress to substitute its foreign policy preferences for those of the president.
Naturally, Dershowitz leaves out any reference to precisely where the Constitution does something like that. Why? Because it doesn't.
According to the Constitution:
The president can negotiate treaties -- which become law if, and only if, the Senate approves them.
The president can appoint ambassadors and a Secretary of State -- who get to assume those positions if, and only if, the Senate approves them.
The president is commander in chief of the armed forces, but only when they're "called into the actual service of the United States." Which generally used to happen when Congress declared war.
Congress gets to "regulate commerce with foreign Nations."
Congress gets to "define and punish Piracies and Felonies committed on the high Seas, and Offenses against the Law of Nations."
Congress gets to "declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water."
Congress gets to "To provide for calling forth the Militia to ... repel Invasions."
And the president's use of money for foreign aid or any other purpose is only allowed "in Consequence of Appropriations made by Law," i.e. by Congress.
Yes, presidents have increasingly seized de facto control over US foreign policy, but nowhere in the Constitution is it so "allocated."