It’s the return of medieval Dispensation’ where the suffering of souls in limbo could be abated by a cash payment.
Also could be used to forgive moral sins that are supposedly unforgivable
Your incorrect on stating “mortal” sins as unforgivable. Mortal sins result in damnation if not repented. There is only one unforgivable sin, blasphemy against the Holy Spirit.
This isn’t an indulgence either.
Both your sentences are false but I am sure you already knew that.