Presidential power in foreign affairs is enhanced greatly by the use of made between the president and other heads of state.

The correct answer here is Executive agreement.

This presidential power allows the president to by pass the Senate and House of Representatives in order to make a deal with a foreign nation. This power given to the president does not go unchecked though, as Congress can vote to not give the appropriate funds to the agreement made between the president and the head of the foreign state whom he has made a deal with.