CREATE TRIGGER tU_Bill FOR Bill AFTER UPDATE AS
DECLARE VARIABLE numrows INTEGER;
BEGIN
  ...
  IF (OLD.bill_id <> NEW.bill_id) THEN
  BEGIN
    select count(*)
      from Billing
      where
        Billing.bill_id = OLD.bill_id into numrows;
    IF (numrows >  0 ) THEN
    BEGIN
      EXCEPTION ERWIN_PARENT_UPDATE_RESTRICT;
    END
  END
  ...

CREATE TRIGGER tU_Bill FOR Bill AFTER UPDATE AS
DECLARE VARIABLE numrows INTEGER;
BEGIN
  ...
  IF (OLD.bill_id <> NEW.bill_id) THEN
  BEGIN
    IF (EXIST(select * from Billing where Billing.bill_id = OLD.bill_id)) THEN
    BEGIN
      EXCEPTION ERWIN_PARENT_UPDATE_RESTRICT;
    END
  END
  ...

CREATE TABLE BILL (
    BILL_ID INTEGER NOT NULL, ...);

CREATE TABLE BILLING (
    BILLING_ID INTEGER NOT NULL,
    BILL_ID INTEGER, ...);

ALTER TABLE BILLING ADD CONSTRAINT FK_BILL FOREIGN KEY (BILL_ID) REFERENCES BILL (BILL_ID) ON UPDATE CASCADE;