I believe he is always negotiating (reconciliation is next week), but you are 100% correct - ultimately he will sign whatever they send him at this point.
He also may be open to using the higher corporate rate to offset deficit spending (although the Senate already resolved the mechanics of that by letting some tax cuts expire). I was shocked when the GOP ran into trouble Thursday night because the Senate parliamentarian stopped a gimmick to cover up the fact that the deficit was rising too much under the plan.
This parliamentarian sure does seem to have some extra-Constitutional power.
Trump has nobody to negotiate with. Neither Ryan or McConnell cares what the President wants or thinks about the matter.
For Trumps part he has to stop making crazy promises. He was in Missouri on Wednesday and he said the tax bill would reduce peoples taxes on an average of $2000 and would result in increases in salaries of $4000. It will do neither, but come 2018 and 2020 you can be sure that the Democrats will be asking people whether their taxes went down and their salaries went up like the President promised.