Most of the major translations say “in the PERSON of Christ” or “with the AUTHORITY of Christ.”
New Living Translation (©2007)
When you forgive this man, I forgive him, too. And when I forgive whatever needs to be forgiven, I do so with Christ’s authority for your benefit,
King James Bible
To whom ye forgive any thing, I [forgive] also: for if I forgave any thing, to whom I forgave [it], for your sakes [forgave I it] in the person of Christ;
American King James Version
To whom you forgive any thing, I forgive also: for if I forgave any thing, to whom I forgave it, for your sakes forgave I it in the person of Christ;
American Standard Version
But to whom ye forgive anything, I forgive also: for what I also have forgiven, if I have forgiven anything, for your sakes have I forgiven it in the presence of Christ;
Bible in Basic English
But if you give forgiveness to anyone, I do the same: for if I have given forgiveness for anything, I have done it because of you, in the person of Christ;
Douay-Rheims Bible
And to whom you have pardoned any thing, I also. For, what I have pardoned, if I have pardoned any thing, for your sakes have I done it in the person of Christ.
Darby Bible Translation
But to whom ye forgive anything, I also; for I also, what I have forgiven, if I have forgiven anything, it is for your sakes in the person of Christ;
English Revised Version
But to whom ye forgive anything, I forgive also: for what I also have forgiven, if I have forgiven anything, for your sakes have I forgiven it in the person of Christ;
Webster’s Bible Translation
To whom ye forgive any thing, I forgive also: for if I forgave any thing, to whom I forgave it, for your sakes I forgave it, in the person of Christ;
Young’s Literal Translation
And to whom ye forgive anything – I also; for I also, if I have forgiven anything, to whom I have forgiven it, because of you – in the person of Christ – I forgive it,