Definition (house-style)
Lutroo (Gr. λυτρόω) is the third and most complete Greek redemption verb. While agorazo describes the purchase act and exagorazo the removal from the market, lutroo means ‘to set free by payment of a price’ — the ransom (lutron) is paid and the slave is given full freedom. In the New Testament it is connected to the redeemer (lutrotes: Acts 7:35) and to release (lutrosis: Luke 1:68; Heb. 9:12). Christ is the great Redeemer who pays the ransom price by his blood and places the captive in complete freedom.
In threefold redemption, lutroo marks full release: the person is not only purchased and removed from the market, but now stands free — without further obligation, unless he voluntarily chooses to remain in service (cf. Ex. 21:5-6).
Author variants
Warnock
Warnock describes lutroo as the third and most complete step:
“But there is yet a third word for redemption: lutroo, meaning ‘to set free by payment of a price.‘”
[Warnock, The Hyssop that Springeth Out of the Wall, hyssop2.html]
Warnock adds the possibility of voluntary eternal servitude, analogous to Ex. 21:2-6 where the freed slave may choose to remain with his master for ever:
“Oh that we, with that freed slave in Israel, might learn that the only true freedom we shall ever know is that freedom which comes to us when we become the willing bondservants for ever of Him who bought us and then set us free.”
[Warnock, hyssop2.html]
For Warnock, the voluntary return to loving service of Christ is the highest expression of redemption: not autonomous freedom as an end in itself, but freedom chosen in order to serve the Redeemer.