PENALTY FOR THE PLAYER WHO DISCARDS A WINNING TILE (Article 97) 

Commentary: If a player discards a tile that completes another player’s Mah Jongg, the discarder is responsible for paying double the value of the winning hand. This penalty remains the same regardless of how many exposures the winning player had or whether they appeared to be in a ready state.

Question from a player:  What is the penalty for discarding the winning tile?  What is the penalty to the discarder when a player has two exposures on their rack? 
 
Answer from the NMJL:

When a player picks their own Mah Jongg tile, all players pay double the value of the hand. For example, if the winning hand is valued at twenty-five cents, each player pays the winner fifty cents.

When a player Mah Jonggs on a tile discarded by another player, only the discarder pays double value and all other players pay single value.

When a player declares Mah Jongg on any hand, concealed or exposed, and no Jokers are part of the hand (exchanged Jokers from an exposure can make the hand Jokerless), the discarder pays winner four times the value of the hand. All other players pay two times the value of the hand. When a player picks own Mah Jongg tile and no Jokers are part of the hand, all players pay four times the value of the hand. Exception: Singles and Pairs Group, as the bonus is built into the value of these hands.

Source:  Mah Jongg Made Easy (2024 page 26)

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Other answers from the NMJL:

“It is not a penalty but a different ruling, that the discarder of the mah jongg tile of the hand to the winner. It was not considered a penalty when East paid or received double the value of the hand.” 

Source for other answers: National Mah Jongg League Bulletin Q&A 1971, pg 5 https://www.nationalmahjonggleague.org