Informal:
* D'accord, à plus tard! (This is the most common and casual way to say it.)
* Ok, on se voit plus tard!
* Bien, à tout à l'heure! (This is more appropriate if you will see the person again soon.)
Formal:
* D'accord, au revoir. (This is more polite and suitable for a more formal setting.)
* Très bien, à plus tard.
You could also use "Bien" (Okay) followed by a more specific goodbye like "bonne journée" (Have a good day) or "bon après-midi" (Good afternoon).
Remember, the best choice depends on your relationship with the person and the situation.