I just got an idea!
How about, to get the wallet, you have to beat the 3 thieves, and after you beat them, you warp to an identical screen with a string of a part of the password needed to access the part of the dungeon with the wallet upgrade, and other things. So, you have to beat all 3 to get the password.
Example:
1st: Open
2nd: Sesa
3rd: me
Password = Open Sesame.
To make the screen with the password, make tiles with the choices for the first part of the password, 1 correct, 3 incorrect. Choice 1 tile= Tilewarp "A" 2="B" etc. Make identical screens, but make the correct first choice go to a second room with
The same setup, but the tiles are the graphic of the second section. The incorrect choice goes to an identical screen, but the tilewarps all go to the next incorrect screen, and this repeats, but after the third screen, a tile pops up with the word, in red, "INCORRECT". The side warp from the side you entered the screen from should take you back to the screen before all this, for the player to try again.
Anyways, the second correct screen has 4 warps, and 1 of them will take you to the third correct screen. Get my point?

Yeah, there's my 200cents.