‘But to whom you forgive anything, I forgive also: for what I also have forgiven, if I have forgiven anything, it is for your sakes in the presence of Christ, so that no advantage may be gained over us by Satan. For we are not ignorant of his devices.'
So now he was ready to completely forgive, because the man had truly repented and was in a state to receive forgiveness, and this was so that Satan might not gain advantage out of the situation. It was good that the man had had to humble himself. But it would not be good if that led to him losing faith and hope. And they had to keep in mind Satan's wiles and devices, which they were surely fully aware of.
He therefore assures them that when they forgive the man, as he has exhorted, he will forgive him as well. (Note that he still leaves the decision in their hands. A pastor who forces his people to accept his will is no pastor). Indeed that for their sakes he has already forgiven the man ‘in the presence of Christ', (probably signifying in prayer before Him), if indeed his forgiveness was necessary, for what mattered most was Christ's forgiveness, and their forgiveness, so that Satan might not have any opportunity to gain any advantage over them.
Paul Demonstrates How Concerned He Had Been But Rejoices That His Ministry Has Been Thoroughly Vindicated (2 Corinthians 2:12-17).
Having dealt with the question as to why he had failed to visit them as he had promised, and what had happened subsequently, Paul brings home to them the relief that had been his when he learned that they had responded positively to his letter. Together with an unexpected opportunity at Troas where God had worked abundantly it had made him recognise that even in the darkest hour God continually leads His people in triumph. God is not defeated by circumstances. They are but stepping stones leading to His further glory.