Question: Is it okay to call a tile for Mah Jongg and then do a Joker exchange to use the Joker to complete the win?

Answer: Once a player calls a tile and declares Mah Jongg, the called tile must be used as the winning tile.  However, there are times when a player calls a tile for an exposure and after completing the exposure, does a joker exchange which results in Mah Jongg. This win would also be considered a “self-picked” win.  

Source:  Mah Jongg Made Easy 2024 page 23 #7