Forgiveness only counts as forgiveness when you could have done otherwise. If there was no consequence you could have imposed, letting it go is simply what was going to happen. Mercy has weight precisely because it is a decision made from a position where something else was possible, and both people usually know it.

If you choose to let something go, be clear with yourself that you are choosing, not merely accepting the inevitable. Say what you could have done and that you are not doing it. That clarity keeps the mercy from reading as weakness, and keeps you from later resenting a generosity you never quite decided on.