Sen. Mike Duffy is considering using the courts to get tens of thousands of dollars in Senate salary he was denied ahead of his trial, his lawyer told CTV News.

Duffy could use the "legal process" to get the income he was denied when the Senate suspended him and two others without pay as they were being investigated by the RCMP for wrongfully claimed expenses. Duffy was found not guilty of all 31 charges against, but lost a year and a half of salary before the suspension was lifted automatically when Parliament dissolved ahead of the last election.

"Absolutely, he's going to try and get it back," Donald Bayne, Duffy's lawyer, said in an interview with CTV's Glen McGregor. Going through the legal system is "being very carefully considered right now, yes."

Duffy is also fighting a Senate committee that has demanded he repay $16,955 in expenses, in part by arguing the Upper Chamber unjustly denied him that $155,867.56 in salary before he had a trial.

Duffy was found not guilty last spring of all 31 charges against him. He was suspended Nov. 5, 2013, and started receiving his salary and benefits again last August, when former prime minister Stephen Harper called the last election.

"This unjust suspension and severe economic and reputational penalty was imposed on a man known to be confronting grave health issues exacerbated by stress," Bayne wrote in a brief to the Senate, released to the media late Wednesday night.

"The Senate now seeks to compound that unjust and oppressive penalty already paid in full by Senator Duffy," Bayne wrote.

The Senate's committee on internal economy, budgets and administration says Duffy has to repay the "non-compliant" expenses, prove they were legitimate, or submit to arbitration. Duffy was one of 30 senators whom the auditor general found were wrongly reimbursed for expenses, with the auditor general contending the expenses actually broke the Senate's rules. The audit was separate from the RCMP investigation that resulted in Duffy's year-long court process.

The judge's decision at the end of the trial, Bayne notes, "criticized Senate leaders for unjustly doing the bidding of the office of the then prime minister." The Crown is not appealing the not-guilty finding.

The expenses the committee wants repaid include $10,000 in payments to a personal trainer whom Duffy says was consulting on seniors' fitness, a $500 payment to an office volunteer, and a $300 payment to a makeup artist hired ahead of a public appearance.

The group of 30 senators identified by Auditor General Michael Ferguson includes Leo Housakos, the current chair of the committee seeking to recover the money from Duffy. It also includes Claude Carignan, the Senate Conservative leader, and David Tkachuk, the Conservative senator who used to chair the internal economy committee. Their expenses add up to $19,766, Bayne says in the brief.

Senate 'acted prudently'

Duffy’s lawyer also argues the other senators captured in the audit were only examined for a two-year period, while the Duffy expenses sought by the committee cover a longer period, with three of them falling outside the audit period used for the other senators. Further, he says, the judge's finding of fact cannot be challenged.

"It is therefore imperative that all senators and the Canadian public know the true facts of the decision of Justice [Charles] Vaillancourt... and the penalty that Senator Duffy has already been unjustly compelled to pay," Bayne wrote. The Senate committee's pursuit of these expenses, plus the severity of Duffy's suspension and lack of salary during the suspension, "is a further compounding of injustice upon injustice.

"Thus, these observations are being concurrently released to all senators and to the media."

In the Senate letter included in documents distributed to reporters, committee clerk Nicole Proulx writes that Duffy's expenses weren't initially subject to the Senate audit because they had already been reviewed in an outside audit after media reports raised questions about his and three other senators' spending.

Despite the court ruling in Duffy's favour, the clerk wrote to Duffy, new information surfaced during the trial and the Senate's chief financial officer reviewed his expenses based on that.

"The conclusion was that if the information had been disclosed or known prior to processing contract or payment, the requests would have been considered non-compliant with the applicable Senate Administrative Rules," Proulx wrote.

The letter is dated June 8 and gave Duffy 10 days to respond.

In a statement released Thursday, Housakos and the committee's deputy chair, Jane Cordy, said the Senate's chief financial officer "acted prudently."

"The Dispute Resolution Process is an independent, arm's length process that ensures senators are not sitting in judgment of other senators. This process is being adhered to in Senator Duffy’s case exactly as it has been for all senators since its implementation," they wrote in the letter.

"The documentation submitted by Senator Duffy through his lawyer meets the deadline requirement established as part of that process and will now be taken into consideration."