[what's up]

But is simple forgiveness of the boy by the neighbor sufficient for the reparation of the wrong? Shouldn't the boy repay the neighbor for the broken window, in addition to asking for forgiveness?

Sure the boy (or sinner) should do restitution or pay back for wrongs done when/if possible.

However, paying back God? Don't think so.

[MEGoody]

However, paying back God? Don't think so.

Agreed. How would one go about doing that anyway? He owns it all - we're just being allowed to use it.

[Aquinasfan]

However, paying back God? Don't think so.

It's not a perfect analogy. That's why I included the formal definition.

An indulgence is the extra-sacramental remission of the temporal punishment due, in God's justice, to sin that has been forgiven, which remission is granted by the Church in the exercise of the power of the keys, through the application of the superabundant merits of Christ and of the saints, and for some just and reasonable motive.

The debt is "payed back" by drawing on the merits of Christ. In a sense, God is paying back Himself, through His Church.