sanofi-aventis (NYSE: SNY) still wants investors shares in Genzyme (Nasdaq: GENZ), but it's not willing to step up the price. The French pharma extended its tender offer to Jan. 21 after not enough shareholders handed over their shares.

And why would they? Genzyme says the offer is too low, and investors seem to agree, having pushed the shares above Sanofi's $69 offer.

One has to wonder who owns the 2.2 million or so shares that have been tendered. The owners of about 1% of Genzyme's shares either don't read what they're signing or aren't smart enough to sell on the open market for $70 and some change.

Clearly, this is a futile exercise. Sanofi has to up its offer or walk away. The latter would likely cause a precipitous drop in Genzyme's share price, and it's the reason I'm not interested in playing this game.

Most of the difference in the values that the two sides have put on the company seems to be over the potential of Genzyme's leukemia-turned-multiple-sclerosis drug Campath. If the drug can make the transition, there's potential for blockbuster sales. But that's a big "if."

The solution is for Sanofi to add a contingent value right (CVR) to its offer. CVRs are the acquisition equivalent to clinical, regulatory, and sales milestones for licensed drugs. The acquirer puts some money upfront but agrees to pay an additional sum should a certain milestone be met. The "buy now, pay if it works" scheme keeps the risk on the shoulders of the sellers rather than the buyers.

BioMarin Pharmaceutical (Nasdaq: BMRN) has done a couple of these deals this year. It bought ZyStor for $22 million upfront and up to an additional $93 million later, and it also purchased LEAD Therapeutics for $18 million upfront and up to an additional $79 million later. Onyx Pharmaceuticals (Nasdaq: ONXX) hedged its bet when it picked up multiple myeloma drug carfilzomib from privately held Proteolix, although it looks like it may have to pay the full amount considering how good the drug looks.

But would Genzyme's shareholders be willing to take a CVR? In the BioMarin and Onyx deals, the sellers were privately held companies. Venture capitalists are probably more willing to take on the added risk of being paid later than public-company shareholders, who often want instant gratification.

Stay tuned, Fools. We're in overtime, but this game seems far from over.

BioMarin Pharmaceutical is a Motley Fool Rule Breakers recommendation. The newsletter is always on the hunt for hot drug stocks and other cutting-edge picks. Click here to see all of our latest discoveries with a free 30-day trial subscription.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool has a disclosure policy.