In a regulatory filing issued yesterday, the U.S. biotech company lays out its case that Sanofi-Aventis (SNY) is playing low-ball with its $69 per share offer.

The filing details a Sept. 20 meeting between Genzyme CEO Henri Termeer and Sanofi chief Chris Viehbacher, which came at the initial urging of Genzyme shareholder Carl Icahn. According to Genzyme, Viehbacher “proposed that the parties agree to a price range of from $69.00 per share to $80.00 per share.”

It goes onto say that Viehbacher indicated a willingness to increase the $69 per share offer, so long as Sanofi could enter into formal talks with Genzyme (which demurred).

In response, a spokesman for the French company today denied that Viehbacher ever put forth a price range. In other words, Sanofi is saying that Genzyme lied in its regulatory filing.

I wasn’t in the room, but tend to believe Genzyme’s side of the story (and not just because I live in Boston). Almost nobody believes Sanofi can get this deal done at $69 per share, so it’s entirely reasonable that Viehbacher would have suggested a pot-sweetener to get the two sides negotiating. The only question now is if public shareholders will believe that $80 per share target, and keep pushing up the price. As of last check, Genzyme shares were trading at nearly $73.