It is written as 'a billion dollars' but is it correct? Shouldn't 'a billion dollar' supposed to be 'billion dollars'?

Rebecca

The phrase "a billion dollars" is correctly used in this fragment. The anchor says "a billion dollars" because she is making the amount a countable number for the viewers to grasp how much was stolen. If she said "billions of dollars" it would be considered an uncountable number and wouldn't emphasize the amount stolen.