This is not 100% definitely true.
When Messi's contract was leaked by der spiegel and mediapart in France, they didn't explicitly state if he gets €70m as a lump sum at the end, or if it's paid out pro-rata, which is more common.
So they've possibly paid him €35m of that 70 already with another €17.5 due around about now for the 19/20 season, and another €17.5 at the end of the deal.
By the way, if those two news organisations felt like leaking the other part of his contract, like the exit clause, we could all be a lot more informed...