A $90,000 cheque from former PMO aide Nigel Wright to Mike Duffy to repay the senator's ineligible expenses may raise questions, but it could be difficult to prove the gift constitutes a criminal offence, according to a long-serving former Law Clerk of Parliament.

Rob Walsh, who was Parliament's top legal adviser until 2012, says in theory, a criminal offence would occur if money was given to a senator in exchange for some kind of benefit.

But in case of the Duffy cheque, Walsh can't see what benefit Wright would have received for repaying Duffy's expenses.

No charges have been laid against either Duffy or Wright, but the RCMP is investigating the circumstances that led to Wright writing a personal cheque after Duffy told him he didn't have the money himself.

Former House of Commons Law Clerk Rob Walsh, at a committee hearing March 16, 2011, was Parliament's top legal adviser before he retired in February 2012. (Sean Kilpatrick/Canadian Press)

RCMP Cpl. Greg Horton states in a court document that a Senate report, meant to be critical of Duffy, was altered in Duffy's favour by the Senate's committee on internal economy to find no fault with his behaviour.

Horton concluded, "I believe there was an agreement between Duffy and Wright involving repayment of the $90,000 and a Senate report that would not be critical of him, constituting an offence of frauds upon the government."

"But what benefit did Duffy give to Wright in exchange for the $90,000?" asks Walsh, who retired in February after 14 years as parliamentary counsel, in an email to CBC News.

What benefit?

"That Duffy make no further media comments? This is not a benefit to Wright," he continued.

"Stop the audit report? Duffy didn't have the power to do this. Edit the committee report? Duffy didn't have the power to do this.

"The funds were given to Duffy to bring the Duffy expenses controversy to an end, like settling a lawsuit. This is not fraud, nor is it breach of trust."

He added, "I don't see any of this supporting criminal charges. It's just self-serving politics."

Duffy has not been charged with any criminal offence. However, he is also being investigated by the RCMP for charging expenses for a home in Ottawa he's owned for years, claiming his primary residence is his P.E.I. cottage. As well, the RCMP is looking at his claims for Senate per diems while on vacation, and while he was working for the Conservative Party during the last election.

Senators Mac Harb and Patrick Brazeau are also the subjects of an RCMP investigation over their expense claims for living expenses in Ottawa. A Senate committee looking into expenses claimed by Pamela Wallin has referred her file to the RCMP as well, though the Mounties have not indicated whether they will investigate.

Each of the senators under scrutiny has said the rules were unclear, or that mistakes were made, or that the rules were followed as they existed at the time of the claims.

Lavigne also said rules were unclear

Former Liberal senator Raymond Lavigne made statements of a similar nature during his trial for breach of trust over false claims he made about travel to Montreal and for having Senate staff cut down trees on his private property during Senate business hours.

Lavigne, who was a senator from 2002 until he retired after his 2011 conviction, submitted mileage claims for trips to Montreal even though he was travelling in his assistant's car, or sometimes not travelling at all. In court he argued the Senate did not have written rules about claims until 2004, and that the claim forms did not specify that the claimant's own car had to be used.

However the judge ruled, "If he [Lavigne] had any uncertainty, he could have asked the Senate administrative staff if he could claim a reimbursement for travel expenses he did not incur."

At trial, the clerk of the Senate at the time, Paul Belisle, testified that even though a consolidated set of administrative rules did not exist, policies were in place that had been explained to Lavigne.

The judge also found that there was no evidence Lavigne was confused about the rules, or did not understand them.

In addition, he cited a Supreme Court ruling that states, "A person is not saved from conviction because he or she believes there is nothing wrong with what she is doing."

Lavigne resigned from the Senate just after he was convicted, thus preserving his pension. He was sentenced to six months in jail followed by six months of house arrest. In August, he was denied early parole.