This article says they dropped it to $850M. It also says they turned down a B$1.5 offer from Viacom around the same time.

I don't think your claims are correct.

This article says that Zuck rejected the $1b offer because he thought it was too low and that the board later rejected a lower number. Not exactly what you said. It specifically says that Zuck believed Facebook to be worth more than $1b. Which is directly counter to the statement "He would have sold for $1b."

Just having a conversation. M & A is one of my favorite topics. Zuck kept changing his mind. Following lines are also from the article. This story is now almost 10 years old so kind of hard to find related stuff.

"Yahoo lowered its offer to $850 million after announcing horrible Q2 earnings. Facebook's board took 10 minutes to reject the lowered offer"

You still haven't provided a source for "He has famously said he would have sold FB if yahoo kept the $1 billion tag.".