Objection 1. It would seem that the rulers of the people cannot dispense from human laws. For the law is established for the "common weal," as Isidore says (Etym. v, 21). But the common good should not be set aside for the private convenience of an individual: because, as the Philosopher says (Ethic. i, 2), "the good of the nation is more godlike than the good of one man." Therefore it seems that a man should not be dispensed from acting in compliance with the general law.
Objection 2. Further, those who are placed over others are commanded as follows (Deuteronomy 1:17): "You shall hear the little as well as the great; neither shall you respect any man's person, because it is the judgment of God." But to allow one man to do that which is equally forbidden to all, seems to be respect of persons. Therefore the rulers of a community cannot grant such dispensations, since this is against a precept of the Divine law.
Objection 3. Further, human law, in order to be just, should accord with the natural and Divine laws: else it would not "foster religion," nor be "helpful to discipline," which is requisite to the nature of law, as laid down by Isidore (Etym. v, 3). But no man can dispense from the Divine and natural laws. Neither, therefore, can he dispense from the human law.
On the contrary, The Apostle says (1 Corinthians 9:17): "A dispensation is committed to me."
I answer that, Dispensation, properly speaking, denotes a measuring out to individuals of some common goods: thus the head of a household is called a dispenser, because to each member of the household he distributes work and necessaries of life in due weight and measure. Accordingly in every community a man is said to dispense, from the very fact that he directs how some general precept is to be fulfilled by each individual. Now it happens at times that a precept, which is conducive to the common weal as a general rule, is not good for a particular individual, or in some particular case, either because it would hinder some greater good, or because it would be the occasion of some evil, as explained above (Question 96, Article 6). But it would be dangerous to leave this to the discretion of each individual, except perhaps by reason of an evident and sudden emergency, as stated above (Question 96, Article 6). Consequently he who is placed over a community is empowered to dispense in a human law that rests upon his authority, so that, when the law fails in its application to persons or circumstances, he may allow the precept of the law not to be observed. If however he grant this permission without any such reason, and of his mere will, he will be an unfaithful or an imprudent dispenser: unfaithful, if he has not the common good in view; imprudent, if he ignores the reasons for granting dispensations. Hence Our Lord says (Luke 12:42): "Who, thinkest thou, is the faithful and wise dispenser [Douay: steward], whom his lord setteth over his family?"
Reply to Objection 1. When a person is dispensed from observing the general law, this should not be done to the prejudice of, but with the intention of benefiting, the common good.
Reply to Objection 2. It is not respect of persons if unequal measures are served out to those who are themselves unequal. Wherefore when the condition of any person requires that he should reasonably receive special treatment, it is not respect of persons if he be the object of special favor.
Reply to Objection 3. Natural law, so far as it contains general precepts, which never fail, does not allow of dispensations. In other precepts, however, which are as conclusions of the general precepts, man sometimes grants a dispensation: for instance, that a loan should not be paid back to the betrayer of his country, or something similar. But to the Divine law each man stands as a private person to the public law to which he is subject. Wherefore just as none can dispense from public human law, except the man from whom the law derives its authority, or his delegate; so, in the precepts of the Divine law, which are from God, none can dispense but God, or the man to whom He may give special power for that purpose.
Source: Summa Theologica (New Advent)