Does saying to someone who harms you with words or deeds, "May Allah forgive you," make you earn a good deed from them or have a bad deed subtracted from you? Is this statement wrong? Is there anything in religion that would make this happen in this life?

There is no Shar'i (Islamic legal) evidence to prove that the supplication of the oppressed causes the transgressor's good deeds to be taken away. However, Islamic law encourages the oppressed to forgive and pardon. If the oppressed is not avenged in this world and does not forgive, then justice will be served for them against the oppressor in the Hereafter by taking away the oppressor's good deeds, or by burdening the oppressor with the sins of the oppressed if the oppressor has no good deeds. The proof for this is the hadith of the Messenger of Allah (peace and blessings be upon him) regarding the bankrupt person on the Day of Resurrection, who comes with many good deeds but has transgressed against the rights of others. His good deeds are taken to compensate for their injustices, and if his good deeds are exhausted, then some of their sins are taken and cast upon him, and then he is cast into the Fire.