The minority assembly government looks set to lose a crucial vote on how it funds public services next year.
Opposition parties say they will vote against the 2007-08 £14.4bn budget unless there is more for education.
But Plaid Cymru invited ministers to negotiate directly with them, breaking with the rest of the opposition.
Plaid's split with the Liberal Democrats and Conservatives appears to offer Labour its best hope of delivering a budget on Wednesday.
There had been suggestions an extra £9m from Chancellor Gordon Brown might have ended the stalemate, if Labour were to devote enough of the extra money to education.
However, all three opposition parties have said that even if the entire sum were to go to education it would still not be enough to persuade them to let the budget through.
The Conservatives have urged Mr Morgan to re-open negotiations even at this late stage.
And the Liberal Democrats have suggested the assembly be recalled from its Christmas break if necessary to get a budget passed.
The vote on the 2007-2008 assembly budget takes place on 13 December.