The Trudeau government is refusing to explain Foreign Affairs Minister Stéphane Dion's latest rationale for not cancelling Ottawa's controversial $15-billion Saudi arms deal, saying it is bound to secrecy and cannot divulge details of the transaction.

Mr. Dion is now saying that Canadian taxpayers may "possibly" be on the hook for huge financial penalties if the Liberals tear up the contract. He warned in answers to the Senate on Thursday that Ottawa could incur "hefty penalties" for walking away from a deal to supply armoured fighting vehicles to Riyadh.

Asked to explain these contract terms that Mr. Dion himself raised – including what the cancellation penalty would be – the minister's department refused to discuss the agreement Friday.

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"For reasons of commercial confidentiality, specific contractual details cannot be shared," Global Affairs spokeswoman Rachna Mishra said.

The Saudi arms deal is no ordinary private-sector transaction. The Canadian government itself is selling the combat vehicles, which are being outfitted with machine guns and anti-tank cannons, to Riyadh. The prime contractor is Ottawa, which brokered the deal through a federal defence export promotion agency called the Canadian Commercial Corporation.

The Liberals are trying to dissociate themselves from this contract – Mr. Dion this week called Saudi Arabia's human-rights record "terrible" – while keeping mum about how they can issue export permits for the deal. Canada's export control rules call for Ottawa to curb weapons shipments to countries with a "persistent record of serious violations of the human rights of their citizens."

Amir Attaran, a University of Ottawa law professor, said the government should not be able to hide the details of a contract it signed from Canadians.

"Surely Canadians have a right to know about a deal that our government is doing on our behalf," Prof. Attaran said.

He called for the Liberals to make the contract public. "By putting the contract before the Commons, there could be a truly informed debate about the cost and benefits of proceeding with, or cancelling, the deal."

Recent polling suggests Canadians feel the arms sale to a country regularly ranked by Freedom House among the "worst of the worst" on human rights is out of step with Canadian values.

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A significant majority of Canadians surveyed recently by Nanos Research believe human rights should trump job creation when it comes to the Saudi arms sale. Nearly six out of 10 polled said they feel it is more important to ensure arms exports go only to countries "that respect human rights" than it is to protect 3,000 jobs that are supported by this deal.

Prof. Attaran questioned how Canada could promise the Saudis to keep the contract private, noting the federal Access to Information Act makes disclosure of third-party information legally possible if it's considered in the public interest.

"Canada would not likely be so foolish as to make a promise [of confidentiality] it might not be able to keep."

NDP foreign affairs critic Hélène Laverdiere called on the Liberals to release details of the Saudi deal.

"Liberals promised to be better than the Conservatives, not act like them," she said.

"Canadians deserve to know the details of this deal and they deserve better from this new government."