How do I verify that only letters or the '$' sign is being used, and to throw an exception if other characters are used

public class Order {
  private String itemName;
  private int priceInCents;
  private String discountCode;

  public Order(String itemName, int priceInCents) {
    this.itemName = itemName;
    this.priceInCents = priceInCents;
  }

  public void applyDiscountCode(String discountCode) {
    this.discountCode = normalizeDiscountCode(discountCode);
  }

  private String normalizeDiscountCode(applyDiscountCode() { 
    for (char letter : discountCode.toCharArray()) {
      if(! Character.isLetter(letter) && letter !='$') {
        throw new IllegalArgumentException("Invalid discount code");
         }
       return discountCode;
       }

   }
}