Best way is to restore a save from earlier in the quest (or before you first interact with Hattori to be extra safe) and play through it again. Alternatively, you can try the following:
1.) Stand close to Hattori and quicksave the game. Exit to the main menu.
2.) Reload the save from the main menu. Try talking to Hattori again. If he still won't respond, transition indoors/outdoors (where Geralt automatically transitions between a walk/jog), then go back in/out. Try talking to him again.
3.) If that doesn't work, transition to the indoor/outdoor location and quicksave again. Exit to the main menu.
4.) Reload and transition indoors/outdoors, then try talking to Hattori again.
That will cover the number of ways the game can load the NPC. If none of those work, then restoring an old save and replaying may be the only way.
Have you used any mods, now or in the past? Especially mods that affect trade inventories can bork NPCs badly at times.