Major League Baseball has sent the Tohoku Rakuten Golden Eagles and their ace Masahiro Tanaka a warning to strictly observe an agreement prohibiting Japanese clubs from receiving any value other than the fee under the new posting system, informed sources told Kyodo News on Monday.

The Eagles received the warning in an email forwarded via Nippon Professional Baseball from the MLB commissioner's office, which has demanded a prompt reply from the club in English.

Tanaka, who is expected to draw a nine-figure contract with a major league team this offseason, raised alarm bells with the commissioner's office after Rakuten said the 25-year-old right-hander told the team he wants to repay the Eagles by "cooperating and donating as much as possible."

Tanaka, who went 24-0 with a 1.27 ERA this season, has said he wants to help improve the Eagles' stadium and facilities for the players and fans.

A provision of the new arrangement between MLB and NPB expressly prohibits a Japanese team from directly or indirectly receiving anything other than the posting fee now capped at $20 million.

The New York Yankees, the Dodgers and Angels of Los Angeles, Arizona Diamondbacks, Chicago Cubs, Seattle Mariners and Texas Rangers are among the teams interested in Tanaka.

The bidding opened Thursday and closes Jan. 24.