Congress doesn't "grant" the president authority, the Constitution does. Congress doesn't have the power to change anything in the Constitution.
You haven't followed my argument. I was critical of Gonzales and the Presidnet for giving the AUMF (which I think you will agree, was a grant from Congress) as the justification for the domestic/terrorist surveillance.
And you are right, Congress doesn't have the power to change the Constitution, and the President doesn't have the power to declare war.