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:

• There is a verbal declaration and the hand has not yet been exposed.
• If only one exposure is made after Mah Jongg is declared and there are no previous exposures on the player’s rack, the declarer can retract their Mah Jongg declaration and use the called tile for an exposure.

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