"tentatively not" is correct and can be used in written English.
You can use it when you want to express uncertainty or doubt about something not happening. For example: "I'm tentatively not going to attend the meeting tomorrow."
Queen Elizabeth started out tentatively, not knowing what to expect.
Williams started tentatively, not moving much, but still broke Muguruza to open the match.
Mr. Shimono led the opening movements of the Berlioz a bit tentatively (not to say slowly), but the orchestra took firm hold of the last two, the "March to the Scaffold" and the "Dream of a Witches' Sabbath," both of which gave ample scope to two of the finest timpanists active, Don Liuzzi, of the Philadelphia Orchestra, and Rainer Seegers, of the Berlin Philharmonic.
And not for nothing, but a few weeks ago we had a sitting U.S. Goddamn President name of Barack Hussein Obama who spoke not tentatively, not neutrally, not cautiously but ADMIRINGLY of the courage of young gay men and women living their lives as full citizens of the United States.
The N.R.A., aware that it could be damaging if Mr. Bush is closely identified with the group, has tentatively decided not to endorse him.
In the New York contest, she and her Democratic allies have tentatively agreed not to spend soft money -- the largely unregulated contributions collected by parties in an effort to circumvent contribution limits -- under an arrangement with Mr. Lazio.
Johnson & Johnson set what is considered the prototype deal in 2007 with Britain's national health system, which had tentatively decided not to pay for the cancer drug Velcade.
PAGE A20 MEDICARE BLOW TO COLON PROCEDURE Medicare has tentatively decided not to pay for virtual colonoscopies, dealing a setback to a technique that some medical experts recommend as a more tolerable alternative to the conventional colonoscopy in screening for cancer.
Hence, in the present study, a substance was tentatively deemed not relevant if it was predicted to contribute less than 10% STU.
Dunham's book, tentatively titled "Not That Kind of Girl: Advice by Lena Dunham," is a collection of essays with an advice motif.
In a way, we suggest tentatively...isn't she almost asking for it?
