The Pay Day money glitch, also known as the Pay Day and Amulet Coin glitch is a natural glitch in Pokémon Ruby and Sapphire.
Holding the Amulet Coin doubles the amount of money earned by Pay Day, however the game still states that the player has picked up the original amount of money in battle, as if the Amulet Coin was never held at all.
For example, a Meowth holding an Amulet Coin uses Pay Day. After the battle ends, the game says that the player picked up (X) amount of coins, though this value is only half of what was actually earned which can be observed by checking your Trainer Card.