God recognizes that Ebed-Melech is no superman, but like everyone, is fearful in the face of mortality. He overcomes his fear, subordinating it to his conviction that Jeremiah is God's spokesman. It is his trust in God that empowers him to show mercy by speaking up for—and then by acting on behalf of—Jeremiah.
God, who "shows no partiality" (Acts 10:34; Romans 2:11), answers in kind, characteristically granting mercy to the merciful, more specifically in this instance, granting mercy to him who "receives a prophet in the name of a prophet." Christ makes it plain that God is resolute in His promise of reciprocity (Matthew 10:41-42).
Using a different image, Solomon says the same: "Cast your bread upon the waters, for you will find it after many days" (Ecclesiastes 11:1).
God is adamant: What goes around, comes around.
Servant of God, Act One: Going Around, Coming Around