After you get rid of the monsters in the gold mine, you return to Fourside.
Receive the Diamond
As you go back to Fourside, the miner's brother drives up in his excavator and gives you the diamond that they found. If this doesn't happen, you will have to go back to the mine and talk to the miner who is standing between the mine entrance and the house. Then go back to Fourside and you will get the Diamond on the way.
Give Miss Fake the Diamond
Go to the Topolla Theater and buy a ticket, and go inside. Go into Miss Fake's office and stand behind her, then use the Diamond on her. She will rip up the Runaway Five's phony contract. The band comes in and thanks you. Go watch their show. Afterward, if you feel like watching Venus's solo performance, you can go back into the theater. Otherwise, go outside.
The Department Store is open now, but you should get ready before you go there. Remove all of Paula's items, but not her weapon or body/arm/other gear. You can give it to Ness and Jeff, or store some with Escargo Express. You can leave things with Paula that you don't think you will need any time soon.
Before you go into the Department Store, make sure that Ness and Jeff have free space in their inventories. Also, if you gave the Franklin Badge to Escargo Express, get it back from them now, and give it to Jeff since he probably has lower HP than Ness. If you don't have any items that can revive unconscious allies (such as Cup of Lifenoodles, Horn of Life, Secret Herb) you might want to take the bus to Threed and use the graveyard secret passage to go to Grapefruit Falls, and from there to Saturn Valley to buy Horns of Life or Secret Herbs. Horns of Life always revive unconscious allies, but Secret Herbs don't always successfully cure unconsciousness.
When you are ready, go into the Department Store. Once you go up the first elevator, don't try to leave the Department Store until you are done shopping. (You can still use the ATM and phone on the first floor, but don't go east past them until you are done shopping.)
Buy some Big Bottle Rockets and the Hyper Beam (if it's stronger than Jeff's current weapon) for Jeff from the Arms Dealer in the room to the left of the food shops, and buy some Teddy Bears from the toy shop for Ness to carry. Don't get the Trick Yo-Yo from the tool shop, because it misses 18.75% of the time. The shops may have stronger armor for Ness or Jeff as well, so be sure to check them all.
Then try to leave the Department Store. During a cutscene, you are told to go to the office on the top floor. You can leave the Department Store whenever you need to. There are dangerous monsters throughout the Department Store. Don't hesitate to use Big Bottle Rockets on them, but make sure that you save at least two or three Big Bottle Rockets for the upcoming boss battle.
Enemies that you will encounter here are:
Defeat Scalding Coffee Cups first, then Mystical Records, then Musicas.
Department Store Spook
Talk to the alien in the office on the top floor to start the boss battle with the Dept. Store Spook. Have Ness use his strongest PSI Rockin and have Jeff use a Big Bottle Rocket on the first turn. After that, keep using Big Bottle Rockets. You might want to have Ness conserve his PP for healing, or you can try using Flash or inflicting sleep, since these have a 50% chance of succeeding.
After you defeat the alien, it says something about Monotoli. Don't bother going to the Monotoli building, because nobody will tell you anything.
Now that you have defeated the Dept. Store Spook, if you walk between the toy store and the toy display on the top floor of the department store, the Photo Man will take your picture.
When you are ready to proceed, visit Jackie's Café in the northwest part of town.