How did roosevelt manage to aid great britain in 1939 and 1940 in various ways short of war?

Though the U.S. was still firmly against joining the war, FDR managed to aid the British through financial loans to their government.  Later he lent those weapons and equipment through the Lend-Lease Act which the British would return after the war. Eventually America entered the war due to the bombing of Pearl Harbor.

the constitution of the united states gave congress the absolute authority over immigration laws