Messi's $20M salary at Inter Miami isn't big if he's going to get over $100M playing SPL. If Al Ahli makes an offer it's going to be hard to reject ecause he's going to get mega money. MLS can't give him salary like SPL can.
Messi already rejected a huge offer from Saudi Arabia before joining Miami. The reason was that he and his family wanted to move to the USA, not to Saudi Arabia. If something changes it will be just because they actually want to leave Miami, not because of the money. If money was the only important thing for Messi he would be already playing in Saudi Arabia. Maybe (probably?) he knows that they will always make a good offer, even when he will be 40 years old, so it's possible that he preferred to play before in the MLS, which is physically more intense than the Saudi league, and he could go to Saudi Arabia just before retiring.