Yes, quadrupling salaries is always the strongest argument, it is difficult to refuse such an offer. I don't wish Liverpool any harm (hope they can break City's hegemony) but it would be interesting to see Klopp's reaction if Salah were poached by the Saudis now. He probably would have started whining even harder about the fact that FIFA and UEFA should do something with Saudi Arabia
Liverpool can still control Mo salah for the next 2 seasons based on his contract, even if he moves next season I don't think it's a problem because Liverpool really have to regenerate so there's nothing to worry about. Regarding salary, FIFA can't do anything because clubs in the Saudi league pay player salaries with company money not debt, another case with Barcelona because it has a debt burden. It doesn't make sense for FIFA to apply rules regarding player salaries while both parties agree, unless there is any indication of entrapment or coercion perhaps that makes sense.