The issue isn't with the league or the clubs, it's Spain's employment law. For soccer players, a salary cut of 50% or more is illegal. In the case of Lewa, it didn't matter because he wasn't a Spanish employee before, but with Messi it meant they had to pay him at least half of what he used to be making, which was already a load of money. This situation came up when Messi was about to leave Barca a few years ago and some people were saying that Messi would play in Barca for free, but besides the obvious reasons why he wouldn't, it was also prevented by law from doing so if he wanted to.