Parliament has already had its say, voting by a large majority to trigger Article 50, earlier this year.
The opportunity to debate and vote on this "deal", relates to any agreement reached with the EU regarding the terms of the UK's departure from the EU on the 19th March 2019.
This may or may not include the "future relationship" ....and it's looking increasingly likely that it may not, looking at the available timescale.
It's very straightforward.
If no agreement is reached with the EU, there will be nothing for parliament to debate and vote on.
If an agreement is reached with the EU, a rejection of that agreement by parliament will effectively mean we will leave without a deal.
There would be b*gger all chance of going back to the EU to try and get a better deal; notwithstanding that the other 27 member States will also be required to vote on whether or not to accept the agreement.
If any of the 27 vote against the "deal", it will be dead in the water anyway.
If all 27 vote in favour of the "deal" and the UK parliament rejects it, it'll be rather academic as not only would the chances of the EU being prepared to renegotiate be extremely slim, but I think it very likely that a firm position will be taken that the 27 have made their decision.
If any agreement between the EU and the UK is reached at the "eleventh hour", or even after the 19th March 2019, it will either be too late for parliament to vote before that date, or the UK will have already ceased to be a member of the EU.
Whatever is agreed between the EU and UK, if anything at all, that will be it ...and realistically, parliament will only have a take it or leave it choice.
That's not a Tory position, but practical reality. Any other UK government taking us through Brexit would find themselves in the same position, whether they liked it or not.