Aquinas did, in fact, endorse the selling of indulgences. IN fact, his own writings (look them up - they are available fairly easily) demonstrate not only an endorsement of the practice, but expended the doctrine to recognize indulgences as effective not only for the living, but for the dead...
So, by his dogma, you could actually “buy” indulgences for your dead father or brother - and shorten their time in Purgatory...
Catholic historians have done much to rewrite history. This isn’t just from a “Protestant” view. A simple study of the remaining writings of early church fathers through current day will demonstrate an historical record (official from the Church) that does not necessarily reflect the reality.
Please read for yourself. Taking Catholic (or Baptist, or Methodist, or Lutheran, or any other for that matter) history or teachings without going to the source is asking to be lead astray.
As a Pastor, I strongly encourage my congregation to not just take what I preach as the “Gospel Truth”, but to go back and read and pray on sermons themselves. This is a huge part of why the Roman Church was so adamantly against common-language translations for centuries (and why early translators were excommunicated and some even burned at the stake). But Catholic History conveniently disregards that chapter in church history as well.
And don’t get me wrong - while I am “Baptist” in my faith and practice, I do not take everything I read and hear about Baptist history or any other teachings without digging in other sources. I am after truth.
Pope St. Pius V, also a Dominican decreed in 1567 that indulgences could not be given for alms-giving. Trent (I don't know when) said pretty much the same.
I have spent about an hour on the search and I couldn't find Aquinas condoning granting indulgences in response to alms-giving. I also couldn't find when the alms/indulgence relationship started up. There's no question that Aquinas is in favor of indulgences generally and discusses them in some detail.
And of course there are still indulgences. My favorite (I don't know if it's still in effect) was the one for giving up smoking. I've given up smoking so many times I'm just going to zip right through Purgatory. (yeah. right.)
I know saying the Rosary in a group is "indulgenced," as they say. It's not a plenary indulgence though.
I am told the whole "so many days/months/years" originally was something along the lines of something like "say X Rosaries in choir and that's as good as fasting for Y days." But later on it sounds like people were thinking seriously in terms of the purgatorial experience as being measurable in days. I really doubt that anyone could find something making that sort of thought de fide. Certainly I have read good Catholic authors who have said that the relationship between purgation and time is highly uncertain.
It is major in MY thinking to remember that the souls in Purgatory are forgiven. Purgatory is as much "therapeutic" as it is retributive. It is a gift to the saved, not a prerequisite for salvation.
I mean, if you're my friend and you break my window, because you're my friend, forgiveness as far as the relationship goes is pretty much assured.
But you yourself will have a need to do what you can to make it up to me, not for the relationship but because you want to repair the damage AND because you want to work on that part of you which leads you to the occasional swinging of baseball bats near other people's windows. Doing the glazing yourself might be wonderful. But if you're as ham-handed as I, you'd better just hire a glazier, and forgo Starbucks for a month ...
I undertake penances not to buy or deserve God's favor, not at all. In fact I think it is a sign of his Love that I sometimes get a notion (and follow through on it) to do something penitential. I trust Him to give me the inestimable, almost inconceivable, favor of somehow incorporating the ridiculously trivial things I do into His saving work, as he may somehow fold our prayers into the gifts he gives us and others.