Protocol for a Joker Exchange (Article ID 75)
Abstract: Once a player begins his hand by picking a tile from the wall, Joker exchange(s) can be done.
Calling a tile and doing a Joker exchange are considered separate actions according to the NMJL. Therefore, if a player calls a tile and wants to do a Joker exchange, they cannot. In other words, the called tile would have to give the player mah jongg without the Joker exchange.
If a player wants a discard to complete an exposure but realizes he must do the Joker exchange to achieve the proper exposure, it isn’t allowed. The exposure would need to be completed without the use of the Joker.
Question from a player: A player discarded a tile that I want to call for mah jongg. However, in order to call that discard, I would need to exchange a symbol tile in my hand for an exposed joker. May I make that exchange and then call for the discard?
Answer from the NMJL: “Players must wait until their turn to redeem “a joker ” or jokers by exchanging a like symbol tile or tiles for the joker(s) taken from the exposure(s). A player must pick or call first before doing the Joker exchange. If you do a Joker exchange before you pick or call your hand could be declared “dead”.”
Source: Mah Jongg Made Easy 2020 page 23 #4
Other answers from the NMJL:
“You must be able to pick up the discard and make your full exposure without the help of the joker. Your exchange always comes after you call for your discard.”
“You must be able to pick up the discard and make your full exposure without the help of the joker. Your exchange comes after you make your exposure.”
Sources for other answers: National Mah Jongg League Bulletin Q&A (2013, 2016,) https://www.nationalmahjonggleague.org