Apparently that isn't the case at all. I can't tell you for sure because I really don't know, but the reports coming from Spain say that Messi can't sign for less than an "X" amount of money established by La Liga, no matter if Messi and Barca come to terms on a lesser fee.
How does that work? I don't know, but apparently that's how the thing works over there.
And about Messi not following the money, it is simple: he would have already agreed to that 1 billion contract if that were the case. If not, why would Messi's father be in Barcelona today, trying to solve the situation and sign with Barca? A Barca team that, we all know, won't be able to even come close to pay Messi that kind of money.