Mike Duffy has been given 15 days to say whether he'll pay back nearly $17,000 in what the Senate considers as inappropriate expenses or face an arbitrator.
In a letter hand-delivered to the Prince Edward Island senator's Ottawa office Friday, the Senate's administrative committee said Duffy can either pay the money or seek arbitration from former Supreme Court justice Ian Binnie.
The letter warns that if Duffy doesn't choose one of the two options by July 23, the money will be deducted from his pay cheque automatically.
Duffy's lawyer, Donald Bayne, argued recently that the senator should not have to repay the money because he was cleared of all criminal charges related to his expenses.
Bayne could not be reached for comment Friday.
But the letter, details of which were obtained by The Canadian Press, makes clear the Senate's committee on internal economy, budgets and administration didn't accept that argument.
And if Sen. Duffy wants to dispute the finding, he can take it up with Binnie, the chair and deputy chair of the committee said in a statement.
"This is an administrative matter between Senator Duffy and Senate finance," said the joint statement from senators Leo Housakos and Jane Cordy.
"As such, the Senate is adhering to the independent dispute resolution process that was implemented in May 2015 in exactly the same manner it has with every other senator."
The committee determined that new information provided by Bayne didn't sway the argument, they said.
"Senate finance has reviewed the documentation submitted by Senator Duffy's legal counsel and has determined there is no mitigating information contained within. The Steering Committee has therefore notified Senator Duffy of the amount owing to the Senate of Canada of $16,955."
The committee said it discovered problems with expense claims filed by Duffy between March 5, 2009 and Sept. 13, 2012, including charges for makeup, a fitness trainer and personal photos.
Duffy was kicked out of the Conservative caucus of then-prime minister Stephen Harper and suspended from the upper chamber at the height of the Senate expenses scandal in 2014 after questions were raised about his living and travel claims.
He remained suspended without pay by the Senate until August 2015 when Harper called the federal election campaign.
Duffy returned to the Senate as a non-affiliated member earlier this year after Justice Charles Vaillancourt cleared him in April of all 31 charges he faced.
Binnie ruled in March that 14 senators who opted for his binding arbitration process must return a total of $177,898 to Senate coffers.
The ineligible expenses were flagged in a report by auditor general Michael Ferguson.