Mike Duffy's lawyer says the P.E.I. senator will not repay nearly $17,000 in expenses that a Senate administrative committee insists he owes because a Canadian court has already ruled them appropriate.

In a letter sent Thursday to the steering committee of the Senate's committee on internal economy, Donald Bayne said the demand for repayment, and a second option to submit to arbitration, amount to an "improper collateral attack on the final judgment and factual findings" of the Ontario Court of Justice.

Justice Charles Vaillancourt cleared Duffy of all 31 charges related to allegations of improper spending in April.

"The ruling is a final legal and factual interpretation that cannot be collaterally attacked by the steering committee or a special arbitrator," Bayne wrote to committee clerk Nicole Proulx. "The dispute resolution process was set up for those senators who never had a court determine the appropriateness of their expenses."

The option of going to an arbitrator was created after the federal Auditor General reviewed the expenses of all senators and found 30 to have claimed inappropriate expenses.

Of the 30, all 23 sitting senators have paid back the expenses or went to arbitration with former Supreme Court justice Ian Binnie and repaid revised amounts. The Senate has said it has retained legal representation to pursue repayment from the remaining seven senators, who had retired from the Senate.

Bayne's letter, obtained by CBC News, is just the latest in a series of salvos that began on June 22, when Proulx wrote to Duffy on behalf of the three-member steering committee to demand repayment of $16,955 in expenses.

The repayment sought from Duffy cover expenses incurred between March 2009 and September 2012 for photographs, makeup and the hiring of a personal trainer — all matters dealt with extensively during the trial and found by the judge to be appropriate.

With the charges of fraud and breach of trust against Senator Patrick Brazeau being withdrawn, the legal portion of the Senate expenses scandal appears to have come to an end without a single conviction. Senator Bob Runciman joins us with his reaction. 19:08

'Looking at all options'

On July 8, the committee gave Duffy 15 days to repay the amount or agree to arbitration. Duffy's refusal to do either now puts the issue back in the hands of the senators.

Bayne, in a brief phone call with CBC News Friday, declined to discuss what might happen next. "We are certainly looking at all options."

Bayne continues to argue that Duffy has already paid a heavy price, including the loss of $155,867.56 in salary during his "unjust and improper suspension" from the Senate, and the inability to count that time towards his pension.

It's not clear whether Duffy and the other suspended senators, Patrick Brazeau and Pamela Wallin, could sue the committee. (Their suspensions ended with last summer's election call.)

All three have now been cleared of any criminal proceedings. The Crown this week withdrew charges against Brazeau. Wallin was never charged despite a lengthy investigation by the RCMP. A fourth senator accused of improper expenses, retired Liberal Mac Harb, also had his criminal proceedings dropped by the Crown.