We have no idea if he even offered his resignation or not or on what terms. Reports I have seen stated that the White House was unwilling to meet his terms for resignation and so Rosenstein made the decision that they would have to fire him.
Regardless, I was simply talking about who leaked the story, which again was almost certainly the White House. I am interested to see if the stories about the WH refusing to meet his terms for resignation were true and if so, what those terms were and if they relate to Mueller.