"Ryu 8 Gaiden" official advises you to refrain from purchasing "Golden Balls" due to a bug, and a bug occurs that prevents you from progressing in the game.

"Ryu 8 Gaiden" official advises you to refrain from purchasing "Golden Balls" due to a bug, and a bug occurs that prevents you from progressing in the game.
 On March 11, Ryu Ga Gotoku Studios reported that the action-adventure "Ryu Ga Gotoku 8 Gaiden: Pirates in Hawaii" had a bug that prevented the game from progressing....

The linked article is in Japanese.