Mueller makes a confidential report to Barr. Barr controls the release to the Congress. He should be smart enough to seed the copies he delvers to the Congress (marked, surreptitiously to show where the inevitable leak to the press came from.)
So, Barr can wait for Trump to return to hand it over to the Congress. Mueller does not have the ability to deliver it directly to Congress.
Weissman will leak it.
He may not have the authority ... but I wouldn't put anything past that weasel.