Here's the position of the Supplement to Thomas Aquinas' Summa Theologica to indulgences.
That would be pretty much the Catholic Church's canon at the last quarter of the 13th century, as the result of the academic discussions of the 50 years before.
A part of it, that might even be useful in an Ars game, is this:
I answer that, Temporal things are subordinate to spiritual matters, since we must make use of temporal things on account of spiritual things. Consequently an indulgence must not be granted for the sake of temporal matters as such, but in so far as they are subordinate to spiritual things: such as the quelling of the Church's enemies, who disturb her peace; or such as the building of a church, of a bridge, and other forms of almsgiving. It is therefore evident that there is no simony in these transactions, since a spiritual thing is exchanged, not for a temporal but for a spiritual commodity.