The former chief of staff to Prime Minister Stephen Harper, who wrote a cheque for $90,000 to pay off suspended Senator Mike Duffy’s expense claims after allegations he illegitimately claimed his P.E.I. summer home as a primary residence, will testify at a criminal trial over the affair that Mr. Harper “gave the go-ahead” for a PMO scheme pressuring Sen. Duffy into an admission he broke Senate rules, Sen. Duffy’s lawyer told a judge Tuesday.