Fox Business reporting is a bit more confusing. They are reporting that Rosenstein offered a verbal resignation, but that he is “on his way to the WH” with the expectation that he will be fired.
He may lose some bennies if he’s fired rather than quits. But how long does it take to scrawl “Screw you, I quit, love Rosenweasel” on a piece of paper with a pen?
If you verbally resign, why go to the WH to be fired? Just mail in your resignation letter.