I've read the English version of the article, and it only reveals this: if United choose not to exercise the option of an additional year, we have to pay 2 million Euros to Cavani as compensation. If Cavani chooses not to exercise the option, then we won't have to pay anything.
The Spanish version reveals another key bit of info (thanks to this Tweet):
Apparently, we want to keep Cavani not only for next season but also for the season after. Cavani, however, is unsure and is considering wanting to leave at the end of this season.
Overall, we're seeing a conflict in the info from different sources. Luckhurst has said that United have yet to commence talks for a renewal (even if Cavani and his entourage are interested). South American sources (includes this info from Llorens and Marsden), however, are saying that Cavani himself is not so keen on renewing his stay with United. Regardless, I think we're heading into a transfer window where we will most likely lose Cavani and will have to replace him.