When God promised Abraham a great reward, Abraham's answer was not gratitude. It was an honest complaint. Gifts without children are not quite gifts. Targum Pseudo-Jonathan on Genesis 15:2 lets us overhear the full weight of what he said.
Lord God, he says, You have given me great blessings, and greater ones still wait in Your presence to be given. But what good is any of it if I pass from the world without a child? My whole house will pass to Eliezer, my steward — bar pharnasath, the son of sustenance, the Targum calls him, because he managed Abraham's household. The same Eliezer, the Targum notes, by whose hand signs were once wrought in Damascus.
That last detail is the Aramaic paraphrase reaching back to an older memory. When Abraham pursued the kings to rescue Lot, his servant went with him and miracles happened. Eliezer is no stranger. He is loyal, capable, even miraculous. And that is precisely why the thought is unbearable. A faithful servant is not a son. The covenant the Lord keeps speaking of needs flesh that belongs to Abraham.
The Maggid hears something bracing in this verse. The father of faith prays by arguing. He does not thank God for what might come; he tells God what is still missing. The Targum preserves the whole uncomfortable prayer — blessings received, blessings withheld, a steward in the wings, and a promise that has not yet come true (Genesis 15:2). Honest grief is also a form of trust.