French website Mediapart reported Saturday that it had obtained a 2006 Libyan document signed by Libya's then-intelligence chief, Moussa Koussa, with the offer.
Sarkozy told France Television Two Monday that the claim was grotesque, and said he would file a lawsuit before the end of the current presidential campaign.
Sarkozy spearheaded Western military intervention that helped drive the Libyan leader from power in a 2011 popular uprising.
France will hold a runoff election Sunday, with Sarkozy trailing socialist candidate Francois Hollande in opinion polls. Hollande emerged as the frontrunner in the April 22 first-round vote, edging the incumbent with 28 percent of the vote to Sarkozy's 27 percent.
