A ransom implies four things: a captive, a captor, a price and a payer. The Christian sources are clear about three of them. The captive is humanity, the price is the life of Christ, and the payer is God. The fourth is where the whole difficulty lies.
One line of early answers named the devil. The reasoning is not as crude as it is sometimes made to sound. If humanity had genuinely handed itself over, then a rescue by sheer force would be a seizure, and God would be acting as a stronger tyrant rather than as a just one. Paying is what distinguishes a redemption from a raid. Writers in this line produced the images the theory is remembered for, above all the hook hidden in the bait, where the adversary reaches for a mortal man and closes on divinity.
The objection to that answer arrived early and from inside. Gregory of Nazianzus asked directly whom such a price could be owed to. If to the devil, then the outrage is that the robber receives not only a ransom but God himself. If to the Father, he asked why the Father should require the blood of the Son when he had not accepted Isaac. Both halves of that question have been repeated ever since, and the second half is now more often aimed at penal substitution than at the ransom view.
Anselm's objection was different and structural. A rebellious creature can hold no rights against the one who made it, so there is no claim to be bought out and the transaction has been misdescribed from the start.
The atlas records the question as open rather than as answered by any of these. What can be said without controversy is that the metaphor is scriptural, that the early church used it heavily, and that no formulation of the recipient ever obtained general assent.