Senator Mike Duffy is rejecting an ultimatum by the Senate's administrative committee, which gave him 15 days to repay $16,955 it says he owes the Senate or face a dispute-resolution process led by former Supreme Court justice Ian Binnie. If he doesn't pay up or allow Mr. Binnie to look at his expenses, the committee says it will deduct the amount from his pay.

The money relates to seven expenses for which Mr. Duffy billed the Senate, including $8 for a personal photo and $10,000 for a personal trainer. All seven items were included among the 31 criminal charges of fraud, breach of trust and bribery that the RCMP laid against Mr. Duffy. After a trial, Ontario Court Justice Charles Vaillancourt found him not guilty of all charges in April. Prosecutors did not appeal the decision.

Mr. Duffy's lawyer, Donald Bayne, said the Senate has no legal right to pursue the senator, in light of Justice Vallancourt's ruling.

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"The ruling is a final legal and factual determination that cannot be collaterally attacked," he said in a letter this week to committee clerk Nicole Proulx. His letter cites no legal authority for that claim. In a June 22 letter, Mr. Bayne cited two cases as authority, but neither case involved the Senate or House of Commons.

He also said Mr. Duffy had been forced to forgo $155,867.56 in pay and benefits during what he called an unjust suspension from the Senate. The demand for $16,955 "compounds the injustice," Mr. Bayne said, adding that Mr. Duffy Duffy will not "'repay' expenses that a Canadian court of law has held were 'appropriate.'"

The committee has said it has the right to insist on repayment. "This is an administrative matter between Senator Duffy and Senate finance," Senators Leo Housakos and Jane Cordy said in a joint statement on July 8.

Mr. Housakos was one of 30 senators found to have questionable expense claims by Auditor-General Michael Ferguson in a report last year. He repaid the money, though he said he disagreed with the findings.

Ned Franks, an emeritus professor in the political studies department at Queen's University, said in an interview that the general principles of parliamentary government "are that the legislature and the courts are separate and the courts don't interfere with what the legislature does. By and large in their own internal proceedings, they're responsible."

He added that to this point, "Mike Duffy has shown a quite astonishing and impressive ability to avoid the consequences of what most people would say are improper actions. I just simply have no idea how that's going to end."

Mr. Ferguson's 2015 report did not review the expenses of Mr. Duffy or two other senators, Patrick Brazeau and Pamela Wallin, because those senators were under investigation by the RCMP. This week, the RCMP said they would drop charges against Mr. Brazeau and former senator Mac Harb. They did not lay charges against Ms. Wallin. Both Mr. Brazeau and Mr. Duffy have said they are considering suing the Senate to recover the pay they lost while suspended.