It’s official. After months of deliberations and just a day after Valentine’s day, the French pharma giant Sanofi-Aventis finalized its love affair with Genzyme Corporation yesterday with a sweetened $20.1 billion buy-out plus payments tied to the success of the US biotech company’s drugs.

As we reported this month, much of the back and forth wrangling over a fair share price for Genzyme, which most analysts pegged at between $75 and $80, stemmed from disagreements regarding the future success of the Cambridge, Massachusetts-based firm’s experimental multiple sclerosis drug Campath. The two companies ultimately settled on a lower immediate price — $74 per share in cash, equivalent to $20.1 billion for now — but together with a so-called ‘contingent value right’ deal, where the pay-out will depend on the sales of Campath and two other medicines. Under optimistic scenarios, that could add $3.8 billion to Genzyme’s sticker price, according to Reuters.

Analysts now expect agreements tied to future earnings to become commonplace among drug makers. “The CVR trend is likely here to stay,” Jason Kantor, an analyst with RBC Capital Markets in San Francisco, told Bloomberg. “It makes [mergers and acquisitions] easier by bridging the valuation gap of buyers and sellers. Sellers want the ‘full value’ of their asset, and buyers don’t want to pay for future risk.”

The Genzyme acquisition is yet another example of a European pharmaceutical company taking over a former darling of the US biotech industry. As Geoffrey Porges, an analyst at the New York–based research firm Sanford C. Bernstein, told me last month: “It raises the question of whether the US pharma companies are being left out in the cold.”