Abdulrahman el-Assir, a Lebanese-born businessman subject to a French arrest warrant issued last November, was questioned on May 30th by Swiss investigators but was not placed in detention, the sources said.
Assir, who is also wanted for questioning in Spain in a money laundering investigation, is a former associate of Franco-Lebanese businessman Ziad Takieddine, who has already been charged with graft in the investigation.
French investigators raided Assir's Paris apartment several months ago but found it empty.
The suspect may soon be questioned in Switzerland by a French judge, the sources said.
The "Karachi Affair", as it is dubbed in the French media, is a complex probe into alleged kickbacks on arms deals.
Investigators are looking into irregularities in the financing of Edouard Balladur's 1995 presidential campaign. Ex-president Nicolas Sarkozy was then-prime minister Balladur's campaign spokesman and budget minister at the time.
Judges suspect Balladur's campaign of receiving illicit "retro-commissions" from the sale of French submarines to Pakistan. Two Sarkozy political aides and a former minister are under formal investigation over the affair.
Judges are also probing claims that a 2002 bombing in Karachi that killed 11 French naval engineers was carried out by Pakistani agents in revenge for the cancellation of bribes secretly promised to officials.
The payment of arms sales commissions was legal in France until 2000, but the payment of kickbacks back to France was and is illegal.
The probe focuses on the 1994 sale of submarines to Pakistan and frigates to Saudi Arabia, in which Assir and Takieddine are believed to have acted as middlemen.
Sarkozy loses his presidential immunity a month after leaving office on May 15th and could be called in for questioning, either as a witness or potentially as a suspect, in the case.
He has repeatedly denied any wrongdoing.