Maybe PSG did not treat him very nice as he expected therefore he decided that PSG would not take any fee from his transfer fee. Because if both parties were in good terms he would had renewed his contract to fetch the club alot of money. I remember last time there was a big disagreement between mbappe and PSG, PSG made a big mistake because they would had sold him last summer to make big profit from him after using him all this while to achieve the success the club has in their league. Mbappe going to Real Madrid is the best move for his career and I do not think he'll end up like Eden Hazard because Mbappe is still in his prime and has alot of contributions he can bring to the team but when Eden Hazard left Chelsea, he was already woreout. Bit still Mbappe needs Real Madrid than Real Madrid needs Mbbape.