No Joker Exchanges after Declaring Mah Jongg (Article ID 60)
Abstract: Once a player calls a tile and declares mah jongg, the called tile must be used as their mah jongg tile. There is no circumstance in which a player can call a discarded tile for mah jongg and do a Joker exchange for the winning tile.
Question from a player: Can I declare mah jongg when my tile is discarded but need to do a joker exchange first?
Answer from the NMJL: “You may not first exchange a symbol tile for a Joker and then use that Joker to call the last discard for either exposure or Mah Jongg. You must make your “call” first.”
Source: Mah Jongg Made Easy 2020 page 23 #8
Other answers from the NMJL:
“After mah jongg is declared there are no picks, discards, or exchanges for the joker. A player can never declare mah jongg on a discard and then exchange the joker.”
Source for other answers: National Mah Jongg League Bulletin Q&A (1967 page 4, 1968 page 4, 1976 page 4, 1982 page 4, 1985, 1991 page 13) https://www.nationalmahjonggleague.org