Student or Learner
Which one in correct?
Person A: I have to go, have a nice day.
Person B: Have a good one.
A:I have to go, have a good one.
B:Have a nice day.
I mean, Can I use "have a good one" befor he/she saying goodbye?
Yes. These expressions are extremely informal. As such, you may use them in any order you wish.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....