Albert Barnes Commentary Jeremiah 39:18

"For I will surely save thee, and thou shalt not fall by the sword, but thy life shall be for a prey unto thee; because thou hast put thy trust in me, saith Jehovah." — Jeremiah 39:18 (ASV)

A prey to you - An unexpected and unlooked-for gain. He had given proof of faith in courageously delivering God’s prophet.