I have sent him back to you

Paul was probably sending Onesimus with another believer who carried this letter.

who is my very heart

Here "heart" is a metonym for a person's emotions. The phrase "who is my very heart" is a metaphor for loving someone. Paul was saying this about Onesimus. AT: "whom I love dearly" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-metaphor)