The Rune Piece is an essential part in Pokémon Sword and Shield if you are looking to earn quick cash. The Piece Rune can help you earn more money by doubling your earnings from fighting other trainers. All you have to do is have a Pokemon keep the coin, and then use it in all of your fights against another trainer. The Rune Piece does not work on Wild Pokémon.
The best place to collect the Rune Piece is just outside of Motostoke, on the outskirts of Motostoke, near Galar Mine 2. Here you'll find a blue sign when you exit the mines and behind the sign you'll find the Rune Piece in a Poké Ball. Just pick it up, then go in your bag and equip the Rune Piece to a Pokémon in your party. It may be a good idea to have a specific Pokémon for Farm Money. Whenever you hire another trainer, use that Pokémon at some point in the fight and your winnings will double.
Doubling your winnings with each encounter with a Trainer is a great way to earn more money, and you can afford to acquire more Poké Balls, Potions, and Revives than you know what to do with. If you want to enjoy the Rune Piece, you can pair it with the Gigantamax Meowth which you can get through the mystery gifts, then use abilities like Pay Day and its Dynamax Gold Rush to earn even more money.
Remember, you need to use the Pokémon that holds the Piece Rune in a battle to get double the money earned, so equip it on a versatile and sturdy Pokémon that you use often. If you are having difficult clashes, you can always change the Rune Piece to another Pokémon.