MLB is going to great lengths to ensure that the Rakuten Golden Eagles don't receive more than the $20 million posting fee in return for Masahiro Tanaka by requesting a written assurance that no outside deals will be made by the Japanese club. By capping the money Rakuten can get at $20 million in the new posting system agreement by NPB and MLB, the Golden Eagles could have potentially looked to other avenues to recoup some of the money they were set to get from Tanaka's exit.
Rakuten's president has come out in opposition of the new agreement and mentioned at a press conference that Tanaka had a desire to donate some of his new fortunes to improve the Golden Eagles' stadium. That obviously set off alarm bells within MLB, who want to make sure that the Japanese team placed no stipulations on the right-hander for requests of money in return for allowing him to come to the United States.
If it is truly Tanaka's wish to donate to his Japanese team, that should be his right with the money he earns from his first contract with an MLB team. The concern is likely over the fact that Japanese teams could virtually blackmail a player into agreeing to give them some amount of money in return for allowing them to post. It's likely difficult to tell whether the idea to donate came from a genuine place or not, so MLB wants to make sure that outside deals aren't being made that could place a player in a tough situation.
Needing to implement rules such as these likely means that the agreement wasn't the best one possible to begin with. It's good that more money will finally go toward the player instead of the Japanese team, but it could also prevent promising Japanese players from being able to come to the United States to play if their team feels like they are getting cheated. Receiving $20 million is better than receiving no money if the player departs in free agency, but they maintain more years of control over the player that way. It will be interesting to monitor how this alters which players decide to make the move from Japan to MLB in the future.