The clear reference to the £1bn promised for Northern Ireland in return for the DUP propping up the UK government raised heckles at the Feile an Phobhail debate.
Asked whether he trusts Mr Johnson, the MP replied: “It’s not a matter of trust. I don’t have to trust Boris. And Boris doesn’t have to trust us. But what we do have to do is business with each other.”
Amid heckles from the audience on the Falls Road, Mr Campbell said his party did business to “bring benefit to every single person in this room”.
“I’ll take a billion criticisms but everybody benefits,” he said.
The panel which also included Sinn Fein leader Mary Lou McDonald and Alliance Party MEP Naomi Long engaged in strong clashes as they debated the backstop, a united Ireland and direct rule.
It was the second year for Mr Varadkar at the Belfast festival. He was criticised in some quarters for opening the event last year due to links between certain events to the Republican movement.
But Mr Varadkar said the criticism had come from some “people down south” and he was now glad to see a cross-party panel taking part.
He was applauded for opening his contribution in Irish but later heckled after arguing Brexit should not be used as a catalyst for a border poll.
Also on stage was Daniel McCrossan (SDLP), Brendan Smith (Fianna Fail) and Doug Beattie (UUP).
Panellists engaged in a Brexit ‘blame game’ with Mr Varadkar reminding the audience that it was the UK which held a referendum.
“That wasn’t our decision but it’s one that we’ve been trying to deal with every since,” he said.
In reply, Ms McDonald warned the Taoiseach that the UK “are pointing their fingers at you, very, very wrongly”.
She added: “This is the Tory boys and girls making a power grab to take that which resides in Brussels back for themselves.”
Ms Long argued that the UK’s dislike of the EU can be traced back to the “ridiculous articles” invented by Boris Johnson during his time as a journalist.
She said Theresa May trigged Article 50 too soon but ultimately argued David Cameron “was the most spectacular irresponsible”.
“What was doubly irresponsibly was having dug the hole for everybody to fall into, just leave.”
Ms McDonald said the Irish government needs to start doing “simple due diligence” to prepare the ground for a 32-county Ireland.
However, Mr Campbell took exception to the Sinn Fein leader’s strident approach saying his “Britishness isn’t up for negotiation, not by her or my Taoiseach or anybody else”.
At one point while discussing social issues, Ms McDonald said the unionist politician should “wise up and come out of the sixteenth century”.
The panel were asked to identify who on stage they most admired as a politician.
Mr Vardkar picked Fianna Fail’s Brendan Smith on the basis that he abstained when he was elected Taoiseach as part of the confidence and supply agreement.
He said the Dail had come together and proved opposing parties can work in the common interest unlike “the mess over in the House of Commons”.
Ms McDonald complimented the Fine Gael leader in her response but settled on Naomi Long who she described as “formidable”.
Gregory Campbell refused to nominate a fellow panellist, saying: “I’ve looked up and down the panel and nah.”