Misko, the Great Bandit
Requirements
It is likely that early in the game you have not collected a single rupee directly; instead, find any character willing to purchase items from you and sell them stuff. Typically minerals (opals, rubies, etc.) are the best for selling; they are highest in value while lowest in usefulness for other things, such as cooking.
Once you have 100 rupees, talk to Domidak and Prissen inside the Dueling Peaks stable repeatedly, until Domidak makes you an offer. This will start the quest in your log.
Walkthrough
Objective 1/End
The cave full of treasure can be found behind a bombable wall near the top of the waterfall. Inside are many treasure chests, including some behind more bombable walls. Even if 100 rupees were paid to start the quest, you will walk away with a greater value in materials and it is worth the trip, especially early on in your adventure, to get some starting wealth. Simply entering the cave will complete the quest.