I have a hunch that it wasn't their preferred currency.

In the first edition of Dahl's novel, Oompa Loompas were Black pygmies Willy Wonka imported from "the deepest and darkest part of the African jungle," according to Jeremy Treglown's Roald Dahl: A Biography. In 1970, the NAACP issued a statement expressing concerns about the racist portrayal of the Oompa Loompas in light of the then-upcoming film. Dahl himself showed sympathy for their stance, and re-imagined them in the 1973 edition as having "golden-brown hair" and "rosy-white" skin.

Despite that change in description, the Oompa Loompas' exploitative origin remained. Wonka smuggled them from their home to work at his factory. They worked tirelessly in exchange for cocoa beans, even as the chocolatier earned real money for their labor. They were prisoners restricted to areas inside the factory. In Charlie and the Chocolate Factory, Willy Wonka learned the tribal language when negotiating a deal with the Oompa Loompas, but he was proud that "they all speak English now."