From SUMMA THEOLOGICA:
newadvent.org/summa/5011.htm#3
Article 4. Whether by the penitent’s permission, a priest may reveal to another a sin which he knows under the seal of confession?
Objection 1. It would seem that a priest may not, by the penitent’s permission, reveal to another a sin which he knows under the seal of confession. For an inferior may not do what his superior may not. Now the Pope cannot give permission for anyone to divulge a sin which he knows through confession. Neither therefore can the penitent give him such a permission.
Objection 2. Further, that which is instituted for the common good of the Church cannot be changed at the will of an individual. Now the secrecy of confession was instituted for the good of the whole Church, in order that men might have greater confidence in approaching the confessional. Therefore the penitent cannot allow the priest to divulge his confession.
Objection 3. Further, if the priest could grant such a permission, this would seem to palliate the wickedness of bad priests, for they might pretend to have received the permission and so they might sin with impunity, which would be unbecoming. Therefore it seems that the penitent cannot grant this permission.
Objection 4. Further, the one to whom this sin is divulged does not know that sin under the seal of confession, so that he may publish a sin which is already blotted out, which is unbecoming. Therefore this permission cannot be granted.
On the contrary, If the sinner consent, a superior may refer him by letter to an inferior priest. Therefore with the consent of the penitent, the priest may reveal a sin of his to another.
Further, whosoever can do a thing of his own authority, can do it through another. But the penitent can by his own authority reveal his sin to another. Therefore he can do it through the priest.
**I answer that **There are two reasons for which the priest is bound to keep a sin secret: first and chiefly, because this very secrecy is essential to the sacrament, in so far as the priest knows that sin, as it is known to God, Whose place he holds in confession: secondly, in order to avoid scandal. Now the penitent can make the priest know, as a man, what he knew before only as God knows it, and he does this when he allows him to divulge it: so that if the priest does reveal it, he does not break the seal of confession. Nevertheless he should beware of giving scandal by revealing the sin, lest he be deemed to have broken the seal.
Reply to Objection 1. The Pope cannot permit a priest to divulge a sin, because he cannot make him to know it as a man, whereas he that has confessed it, can.
Reply to Objection 2. When that is told which was known through another source, that which is instituted for the common good is not done away with, because the seal of confession is not broken.
Reply to Objection 3. This does not bestow impunity on wicked priests, because they are in danger of having to prove that they had the penitent’s permission to reveal the sin, if they should be accused of the contrary.
Reply to Objection 4. He that is informed of a sin through the priest with the penitent’s consent, shares in an act of the priest’s, so that the same applies to him as to an interpreter, unless perchance the penitent wish him to know it unconditionally and freely.