§1. A reduction of the obligations of Masses, to be made only for a just and necessary cause, is reserved to the diocesan bishop and to the supreme moderator of a clerical institute of consecrated life or a society of apostolic life.
§2. With regard to Masses independently founded in legacies, the diocesan bishop has the power, because of diminished revenues and for as long as the cause exists, to reduce the obligations to the level of offering legitimately established in the diocese, provided that there is no one obliged to increase the offering who can effectively be made to do so.
§3. The diocesan bishop also has the power to reduce the obligations or legacies of Masses binding an ecclesiastical institute if the revenue has become insufficient to pursue appropriately the proper purpose of the institute.
§4. The supreme moderator of a clerical institute of consecrated life or a society of apostolic life possesses the same powers mentioned in §§2 and 3.
Source: Code of Canon Law (Vatican.va)