'Reward' in the Bible

Now may the Lord show you true kindness! I also will reward you, because you have done this deed.

Ahimaaz the son of Zadok again spoke to Joab, "Whatever happens, let me go after the Cushite." But Joab said, "Why is it that you want to go, my son? You have no good news that will bring you a reward."

I will cross the Jordan with the king and go a short distance. Why should the king reward me in this way?