The government's offer for extra money as part of the deal to restore Stormont "falls way short" of what was promised, Finance Minister Conor Murphy has said.
Mr Murphy was speaking after the Stormont parties met Secretary of State Julian Smith to discuss how much will be allocated.
The finance minister refused to comment on how much exactly had been proposed.
Earlier the prime minister said the government had made "huge commitments" as part of the deal.
The British and Irish prime ministers were in Belfast on Monday to mark the return of devolution after a three-year impasse.
Speaking to the media, Mr Johnson did not state how much money would be provided to support the deal, saying it was not about money but leadership.
On Monday evening Mr Murphy said: "As far as I'm concerned the conversation hasn't ended, there's still work to be done.
"We have to analyse the verbal figures that were given to us tonight by the secretary of state, but my initial read of them is they fall way short and I wouldn't tend to accept that."
He said the government had made commitments to the Stormont parties.
"They can't come today and congratulate us for living up to our commitments and then not live up to their own," he said.
Mr Murphy had previously said more than £1.5bn was needed.
'Conversation about funding'
The prime minister met the new executive ministers on Monday morning, having been greeted by First Minister Arlene Foster and Deputy First Minister Michelle O'Neill.
Speaking afterwards, Mr Johnson said the government would "strongly support" the new power-sharing executive.
"What's so great about today is, as I say, that Northern Ireland politicians have put aside differences, stepped up to the plate and shown leadership," he added.
The prime minister said this was a chance to "deliver on the priorities of the people" in terms of health, education and crime fighting.
He acknowledged that there was a "certain amount of conversation about funding" and whether the government was going to be supportive.
Mr Johnson said the government was making "huge commitments" for health.
"Yes of course we are going to be supportive, but it's not just about money," he said.
"We are listening very carefully and will certainly do everything we can to support."
Taoiseach (Irish Prime Minister) Leo Varadkar said the Good Friday Agreement "is working again".
"North-south cooperation is going to resume. We are going to beef up and deepen cooperation."
First Minister Arlene Foster said the prime minister must "step up to the plate" and deliver what the government has promised in extra funding for Northern Ireland.
"We need significant and sustained investment, not just this year but over a number of years. This is crucial in ensuring transformation in areas such as health and also our road and water infrastructures," she said.
Mrs Foster also said the possibility of water charges being introduced as a means of raising revenue was not supported by anyone in the executive.
Deputy First Minister Michelle O'Neill said she had impressed upon the two prime ministers "the importance of coming through with the funding promised as part of the deal".
"All executive ministers are committed to working together to tackle some very serious issues in our society and across public services but, quite simply, we need the money to make it happen.
"We have done our bit and I look forward to the fulfilment of the commitments made by the two governments to let us get to work."
