Microsoft has been looking for some time to unload Razorfish, the digital ad agency that it acquired as part of its $6 billion acquisition of aQuantive two years ago. Now the pack of potential suitors appears to be narrowing down.

French ad agency Publicis is currently in the lead to buy Razorfish, and has bid between $500 million and $600 million for the agency, according to a report today in the Wall Street Journal. Microsoft wants the buyer of Razorfish to commit to purchasing "hundreds of millions of dollars" in ads across its various web properties, an issue that's now being "hammered out" by Microsoft and Publicis, the Journal reports.

That kind of ad commitment -- if Microsoft can nail it down -- could boost Microsoft's newly minted partnership with Yahoo as it takes on Google in the internet search market. According to the Journal, Japanese ad giant Dentsu also made a bid for Razorfish, and Omnicom Group and WPP expressed early interest in the agency.

Many observers believe Microsoft was primarily interested in aQuantive's technology units, the online advertising platform Atlas and website ad network DRIVEpm, and picked up Razorfish as part of the deal. Razorfish, which has been hit hard by the economic downturn as clients cut back on ad budgets, has undergone several rounds of layoffs over the past year.