There is a massive amount of daylight between what Clinton told Netanyahu and what Netanyahu told his cabinet. He can't keep up that pretense for long, and so - coward that he is - he is planning to blame the USA and then jump ship i.e. he is demanding that letter so that he can shout: "That wasn't what I was offered! The Deal Is Off!!!"
Egypt says verdict in Al-Jazeera case expected Saturday (AP)
from the article: Netanyahu awaiting written offer from Obama on settlement freeze