As I understand the latest plan for a 'Final vote' put forward by David Davis:
Once all the negotiations are complete, the results will be put to Parliament, and if the 'Remain' side manage to vote it down, we leave on WTO terms.
Similarly, if we increase our offer for a 'divorce settlement', negotiations can proceed immediately. If we don't then get enough movement on our other requirements (to justify the new payment), then when the vote comes the 'Leave' side can vote it down, and we leave on WTO terms (with no 'Divorce settlement').

It looks good to me.