Problem with the logic of karma

Ok, so i am kind of an outsider on this topic, so i assume i got something basic wrong. But even if thats not the case, please try to explain this to me: As i understand it at this point, Karma works somewhat like this: Person A has bad/negative karma so Person B does something bad to them. Through this, Person As negative karma is quasi removed and Person B aquires negative karma for their bad deed. Reaching nirvana requires one to reach karmic neutrality, having neither positive nor negative karma. This setup leads me to one conclusion: Once basically everyone has reached nirvana, in the end there has to be one individual being that holds all the combined karma, and this one being has thus no chance of ever reaching neutrality and thus nirvana. Now, i am pretty sure something in my setup is false, so can someone please explain it to me?