Can't create trigger in MySQL to make a simple audit
; at the end of INSERT statement is lost.

Use

CREATE TRIGGER log_sales_updates
AFTER UPDATE
ON sales
FOR EACH ROW
Insert into audit_log ( sales_id,     previous_amount,  new_amount) 
VALUES                ( NEW.sales_id, OLD.sales_amount, NEW.sales_amount) ;


DELIMITER reassign is excess for single-statement trigger.