The 4th council of the Lateran 1215 indeed determines in constitution 60:
From the complaints which have reached us from bishops in various parts of the world, we have come to know of serious and great excesses of certain abbots who, not content with the boundaries of their own authority, stretch out their hands to things belonging to the episcopal dignity : hearing matrimonial cases, enjoining public penances, even granting letters of indulgences and like presumptions. It sometimes happens from this that episcopal authority is cheapened in the eyes of many. Wishing therefore to provide for both the dignity of bishops and the well-being of abbots in these matters, we strictly forbid by this present decree any abbot to reach out for such things, if he wishes to avoid danger for himself, unless by chance any of them can defend himself by a special concession or some other legitimate reason in respect of such things.
In constitution 62 it states:
Let those who are sent to seek alms be modest and discreet, and let them not stay in taverns or other unsuitable places or incur useless or excessive expenses, being careful above all not to wear the garb of false religion. Moreover, because the keys of the church are brought into contempt and satisfaction through penance loses its force through indiscriminate and excessive indulgences, which certain prelates of churches do not fear to grant, we therefore decree that when a basilica is dedicated, the indulgence shall not be for more than one year, whether it is dedicated by one bishop or by more than one, and for the anniversary of the dedication the remission of penances imposed is not to exceed forty days. We order that the letters of indulgence, which are granted for various reasons at different times, are to fix this number of days, since the Roman pontiff himself, who possesses the plenitude of power, is accustomed to observe this moderation in such things.
About indulgences in early 13th century this does not indicate,
Quite to the contrary, it indicates then common abuses by abbots and bishops, which the council publicly identifies, denounces and sets to rectify. We know, that it did not succeed in the long run to counter the erosion of the Church's repute - especially once the Pope needed further resources himself.
An abbot 'selling' indulgences and dodging problems after 1215 might still be an interesting NPC (or PC?) in an Ars Magica saga.
Not quite. The Supplement does draw a clear line before simony.
We have those in the objections the Supplement lists before tackling them.