I understand Aurel Kolnai has a paper on this topic. I haven't read it. But the paradox has been put to me as follows in conversation.
It is morally objectionable to forgive those who will not admit wrongdoing, show no remorse, make no amends, do not pay restitution, etc. But if forgiveness is made conditional upon the doing of these things, then what is to forgive? Conditional forgiveness is not forgiveness. That is the gist of the putative paradox, assuming I have understood it.
This is something I need to explore, but off the top of my head I fail to see a problem. The first limb strikes me as self-evident: it is indeed morally objectionable to forgive those who will not admit wrongdoing, etc. But I reject the second limb. I admit that once the miscreant has paid his debt, he is morally in the clear. His guilt has been removed. But I can still forgive him because forgiveness does not take away guilt, it merely alters the attitude of the one violated to the one who violated him.
Suppose you take money from my wallet without my permission. I catch you at it and express my moral objection. You give me back my money and apologize for having taken it. I forgive you. My forgiving you makes perfect sense even though you have made restitution and have apologized. For I might not have forgiven you: I might have told you go to hell and get out of my life for good.
By forgiving you, I freely abandon the justified negative attitude toward you that resulted from your bad behavior. This works a salutary change in me, but it also does you good, for now you are restored to my good graces and our mutual relations become once again amicable.
So I see no paradox. The first limb is self-evidently true while the second is false. Only conditional forgiveness is genuine forgiveness.
It is of course possible that I am not thinking deeply enough!