Objection 1. It would seem that a man cannot use the keys in respect of a superior. For every sacramental act requires its proper matter. Now the proper matter for the use of the keys, is a person who is subject, as stated above (Question 19, Article 6). Therefore a priest cannot use the keys in respect of one who is not his subject.
Objection 2. Further, the Church Militant is an image of the Church Triumphant. Now in the heavenly Church an inferior angel never cleanses, enlightens or perfects a higher angel. Therefore neither can an inferior priest exercise on a superior a hierarchical action such as absolution.
Objection 3. Further, the judgment of Penance should be better regulated than the judgment of an external court. Now in the external court an inferior cannot excommunicate or absolve his superior. Therefore, seemingly, neither can he do so in the penitential court.
On the contrary, The higher prelate is also "compassed with infirmity," and may happen to sin. Now the power of the keys is the remedy for sin. Therefore, since he cannot use the key on himself, for he cannot be both judge and accused at the same time, it seems that an inferior can use the power of the keys on him.
Further, absolution which is given through the power of the keys, is ordained to the reception of the Eucharist. But an inferior can give Communion to his superior, if the latter asks him to. Therefore he can use the power of the keys on him if he submit to him.
I answer that, The power of the keys, considered in itself, is applicable to all, as stated above (Article 2): and that a priest is unable to use the keys on some particular person is due to his power being limited to certain individuals. Therefore he who limited his power can extend it to whom he wills, so that he can give him power over himself, although he cannot use the power of the keys on himself, because this power requires to be exercised on a subject, and therefore on someone else, for no man can be subject to himself.
Reply to Objection 1. Although the bishop whom a simple priest absolves is his superior absolutely speaking, yet he is beneath him in so far as he submits himself as a sinner to him.
Reply to Objection 2. In the angels there can be no defect by reason of which the higher angel can submit to the lower, such as there can happen to be among men; and so there is no comparison.
Reply to Objection 3. External judgment is according to men, whereas the judgment of confession is according to God, in Whose sight a man is lessened by sinning, which is not the case in human prelacy. Therefore just as in external judgment no man can pass sentence of excommunication on himself, so neither can he empower another to excommunicate him. On the other hand, in the tribunal of conscience he can give another the power to absolve him, though he cannot use that power himself.
It may also be replied that absolution in the tribunal of the confessional belongs principally to the power of the keys and consequently to the power of jurisdiction, whereas excommunication regards jurisdiction exclusively. And, as to the power of orders, all are equal, but not as to jurisdiction. Wherefore there is no comparison.
Source: Summa Theologica (New Advent)