dinero-proxy-a.js

const nullableDinero = ({ amount, ...rest }) => {
  if (amount === null) {
    return nullDinero;
  }

  let realDinero = Dinero({ amount, ...rest });
  let wrapper = Object.create(realDinero);  // delegate to realDinero by default

  for (let method of ['add', 'subtract']) { // extend specific methods
    wrapper[method] = other => other === nullDinero
      ? nullDinero
      : realDinero[method].call(wrapper, other);
  }

  return wrapper;
};

dinero-proxy-b.js

const nullableDinero = ({ amount, ...rest }) => {
  if (amount === null) {
    return nullDinero;
  }

  let realDinero = Dinero({ amount, ...rest });

  let wrappedMethods = ['add', 'subtract'];
  let passthroughProperties = Object.getOwnPropertyNames(realDinero)
    .filter(name => !wrappedMethods.includes(name));

  let proxy = Object.create(
    Object.getPrototypeOf(realDinero),
    passthroughProperties.map(name => Object.getOwnPropertyDescriptor(name))
  );

  for (let method of wrappedMethods) {
    proxy[method] = other => other === nullDinero
      ? nullDinero
      : realDinero[method].call(proxy, other);
  }

  return proxy;
};