Question: If a player declares Mah Jongg and then realizes they don’t have Mah Jongg, can they change their mind?

Answer:  A player can retract their Mah Jongg declaration if:

1. There is a verbal declaration and the hand has not yet been exposed.
2. Mah Jongg is declared on a discard that has been placed on the declarer’s rack, or the declarer has taken tiles from their rack and realizes they don’t have Mah Jongg but can use the called tile for an exposure.

Source: Mah Jongg Made Easy 2024, pg. 21, #2 & #21, #3