A dollar four eighty

what is "a dollar four eighty"----------an older client of mine says it all the time----"i owe you a dollar four eighty. thank you

It's a humorous use of doubletalk. Next time, reply "no, you actually owe me a dollar seven fifty-six."

The slang word "buck" is usually used in place of the word "dollar" when amounts of this type are discussed. Google turns up 417 hits on "a buck three eighty", which is the number I've heard most frequently, vs. only 6 for "a dollar three eighty" and 41 for "a buck three fifty". I have no idea why one amount seems more popular than another.